If Mage Water is #1 for mantle of oceans. Also Blizzard I believe is the best AOE spell.

Fire is pretty powerful.

For me (I usually make Mage sovs)

Water

Fire

Air

Life

Earth

Death

Life has an absurdly powerful spell, and it's available right at tier 1: Sovereign's Call. In a game where you win by outgrowing your opponents and the typical maximum growth rate of a city is +3/turn, the ability to add another +2 on top of that is extremely good. It's very terrain-dependent, of course, because without high grain cities you'll have trouble creating enough food to keep up with the ridiculous rate your cities are booming at. Sovereign's Call is probably the main reason Empires can't just steamroll Kingdoms because it gives Kingdoms the strategic advantage of being allowed to just defend - even if the Kingdoms have fewer cities they'll be much better developed so they can still keep up. That forces the Empires to attack the Kingdom players or fall hopelessly behind, and combat extremely favours the defender in this game so that's an uphill battle all the way despite the superior destructive force of the Empire players.

Lightning bolt (Aeromancy?), for high level sovereign, is a very powerful spell. I tend to see it sometimes as a Win button.

- It is very cheap and (lightning) rarely resistive, plus there is no cooldown (yeah!).

- You can cast it in a line of 5, meaning with proper positioning, you can hit 5 units.

Imho this thread shows that the magic is interestingly set, with multiple ways to victory and preferences. No nerfs needed

I do actually use slow a lot and I thought I had it in the list, but I guess I overlooked that one not from lack of playing it just simple oversight.

Kill doesn't do a lot for me.  I can see the value of Kill in the main quest and vs wildlands bosses, but I guess I don't play this way because it is too cheesy for me.  I would rather shrink and slow it and then kill it the regular way.  I beat the master quest with Kill once before and I was tremendously underwhelmed.  

One drawback from damage spells is that they force you to play Mage.  Yes, Mage probably is the best type, but it just again greatly reduces your versatility.  Most other schools you can play decently with just Brilliant regardless of what class you choose, but damage magic really wants all its users to be Mages.  Further it wants those mages to invest in +% damage traits rather that investing in other schools of magic.  You really have to pursue damage magic to the exclusion of all else if you want your sovereign to have really amped up damage magic.

If you evaluate schools of magic by how good they are with minimum investment, Fire suffers pretty badly.

If you evaluate schools of magic by how they play in the hands of non-mages, Fire again suffers pretty badly.

Rolling Delin for your wildlands hurts Fire Mages more than any other wildland with any other magic type.  Rolling Quendar as one of your opponents hurts Fire Mages more than any other opponent race hurts any other magic type.

It seems like to me the only way to put Fire anything other than last overall is to seriously underrate what the other schools are capable of doing.

In short: Air (2nd level) to begin with. Tutelage is THE spell to level up your heroes more quickly.

BTW I just love the AI tweaks of 1.4 !

1. Water - Blizzard, mantle of oceans, Vet's Howl

2. Fire -  Firestorm, fireball, flame dart

3. Air - lightning spells, evade

4. Life - Heal, well spring, cloud walk, shrink, growth

5. Earth -  move earth, summon beast

6. Death - blind, death - otherwise "meh"

 can't really rank death magic since i rarely play empires. my ranking for the other schools:

1. Life - no contest. healing spells are just too good, and shrink is a very potent debuff (possibly the best debuff in the game, except for a  heavily water shard buffed slow). sov's call is probably the single best city enchant (IMO) and regeneration is one of the most powerful unit buffs (full heal between seasons) call to arms, crusade and deathward are just icing on the cake (if you have too much mana in the end game)

2. Water. Slow is awesome, pandemonium is probably the most undervalued AoE spell (no cast time if you champ has the Savant trait; it's a bit chaotic which makes it unreliable, but both the daze and slow results are very useful; the damage result is fairly useless though); mantle of oceans is great, blizzard is a really deadly spell vs. multi figure units (unlike fireball, blizzard deals damage per figure, which can be rather crazy vs. 6 figure companies); freeze is probably the best strategic spell throughout the game (tied with Earth II tremor - good synergy if you have both). tidal wave and vetrars howl can be very deadly on the strategic layer.

3. Air. tutelage for heroes, haste for early game, aura of grace probably the strongest fortress troop enchantment; titans breath is really nice for tactical (AoE knockback/knockdown); windwalk is a great strategic mobility spell. tornado is still a very good strategic spell (used to be my favorite strategic spell when it was cheaper; now at 150 mana it's still worth using sometimes). if i have way too much mana, the repeatable +1 init spell is also fun to create a superhero.

4. Earth. tremor, nature's grace (? not sure about the name - the troop buff that gives elemental resistance) and hammers are very useful; set in stone is good for quickly building stuff in newly founded cities; earthquake can be devastating, though too expensive to really spam it (most of the time); giantform and diamondskin can turn a high level warrior or assassin hero  into a truly unstoppable wreckingball. 

5. Fire. it's great for damage spellcasters, but i rarely use those (prefer defenders or warriors backed up by healer/supporter mages/commanders). the fire aura for troops is very good (IMO it's the second best after the +init buff -if i want to play with elite troops, those two are mandatory) you only need fire 1 for the buff, though - if you don't use a damage spellcaster champ, there's little reason to go beyond fire 1. maybe fire 2 for the +fire damage buff for heroes if you have lots of fire shards. fireball is pretty strong on a specialized fire caster. with all the evorker traits and savent, you can burn down full armies before they even get close.

Sacrifice doesn't do anything at all to units. You might be thinking of Cull the Weak (kills a regular unit to heal 20 health on the caster and generate 20 mana) or Infernal Covenant (permanently kills a champion to summon a [champion's level + 7]th level Death Demon who sticks around for the duration of the battle). Sacrifice converts half of the population of one of your cities into mana at a 1:1 ratio. Since there are points in city development where the population really doesn't matter, this is more or less a source of free mana (and using it on level 5 cities has little real cost associated with it, unless you really wanted to use that city for churning out Pioneers). The only three things that Sacrifice really competes with are Blood Curse (sacrifice city population to increase the caster's health), city growth (which doesn't matter if you're casting it on a city which has no reasonable prospect of reaching the next level in a reasonable time period or which is already level 5), and Pioneer training (but since you're probably targeting a high-population city, and probably aren't doing this terribly frequently, shouldn't cause too much difficulty).

As far as why Fire is attractive - it's a minimal investment tactical damage magic school. Everything you need is available in the first three ranks, and you can pick up the first two on sovereign creation if you wanted to have it; Fire III is available by level 5, and unlike Anointed by Fire requires neither a 1-mana-per-turn upkeep nor any specific research path. Even though Fire Resistance and Fire Immunity tends to be more common than other elemental resistances and immunities, it isn't common enough in most circumstances that Fire damage isn't worth having. If you're not interested in playing tactical damage spellcasters, then Fire isn't for you. If you are, then it's your go-to choice for a minimum investment damage school. Fireball isn't the army killer that Blizzard is, but it's faster to cast and easier to get, which makes it more likely that it won't be counterspelled (and it can be made impossible to counterspell with Savant, unlike Blizzard). The fourth and fifth ranks aren't that attractive, but they also don't really need to be if what you're looking for is a minimum-investment damage school, since Flame Dart and Fireball are good spells available very early on (though they probably won't be worth casting except in extremis until at least the later part of the early game, due to mana issues).

Fireball has its uses. Killing ranged units before they can damage or kill your heroes or ranged units springs to mind. Its particularly good against Shrills or Darkling Shamen for this, I've used it to clear everything except the boss from a Wildlands before,  meaning I could pick up all the goodie huts fairly early in the game. Causing damage to very high armour units if you don't have Curse is also a reasonable use. However it's quite expensive, and Blizzard wins hands-down as an AI army killer.

Having Blizzard countered is annoying, but I've only had the AI counter me a few times; if you have a spellcaster with high enough initiative then you may be able to time it in the initiative list to go off before the enemy spellcaster gets a turn. Interestingly Command does not work. Your spellcaster immediately gets an extra turn to do something else, but the spell itself keeps its previous position. However in practice being countered is rarely a problem if you have a high enough initiative; Haste on you and Slow on the enemy spellcaster obviously help. Sometimes you can also wait for the enemy spellcaster to start casting a multi-turn spell of their own.

Incidentally has anyone else noticed enemy spellcasters casting more spells since 1.4? In my last battle once or twice I thought the enemy hero should actually have waded in and done some damage rather than casting yet more buffs and debuffs, but in general this part does seem to have improved a lot. High level enemy spellcasters will now do something other than cast Burning Blade and hit you with their sword.

You're ignoring my point. If you're interested in playing a tactical DAMAGE mage, then you're going to be picking up the Evoker traits and Savant anyways, and Fire II and III are at most a two-level investment (or a 1-level investment if you started with Fire II). And unlike Anointed by Fire, having Fire III on a champion doesn't require 1 mana per turn, which depending on shard distribution and essence availability can potentially be a concern into the later stages of the mid game, and possibly even the end game (yes, you'd have to be really unlucky, but then again I've played on a huge map where only three of my 14 cities had any essence at all because there just weren't that many essence locations to start with, and most of the remaining available land also didn't have any available essence yields).

Neither Haste nor Slow is a damage spell; Blizzard requires a minimum of two levels invested into Water Magic and isn't a versatile spell unless you consider a big hammer of army-smiting as the solution to all your problems, and is additionally worse than Fireball if we're targeting single-figure units or enemies with a decent chance of successfully casting Counterspell. Chaos and Pandemonium, the other spells from Water Magic that can deal damage, are unreliable damage spells because they aren't guaranteed to deal damage and moreover the damage that they deal isn't particularly good (this doesn't make them bad support spells; as Azunai_ said, the Daze and Stun effects that these can apply are rather useful, but these aren't good damage spells). Air Magic tactical damage spells aren't that good - Storm hits a random enemy, and Thunderstrike puts the caster in melee range if you want it to deal any damage - so Air Magic isn't something to turn to as a damage mage, and it additionally requires a minimum of 1 level invested in it to have any damage spells, with a required minimum level of 5, because the first damage spell in Air is available at Air III. For the same amount of investment in Fire, you've already gotten three damage spells - Burning Hands (which has all the problems of Thunderstrike for a primary spellcaster, but is available much sooner and is nearly four times less expensive to cast; at 10 mana per cast and dealing 10 fire damage to up to three adjacent targets, it's additionally possibly worth casting in the early part of the game), Flame Dart (which isn't worth casting early because its primary damage is a multiple of your level and it costs twice as much as Burning Hands to cast), and Fireball (which, though a little too expensive to rely on in the early game, can be used to shift the balance of power in one or maybe two early battles, or as a last resort if you run into a fight you can't handle with just your regular army).

Earth Magic tactical damage spells are quite bad - Shockwave deals less only 55% of the damage that Fireball does (though at no casting time regardless of Savant), requires your caster to be in melee range of all of the targets (who are ideally surrounding the caster - if you think this is an acceptable position for a primary spellcaster, I think you need to reevaluate your tactics), and costs 20% more mana to cast than Fireball does, while Fracture deals damage equal to 25% of the current health of the target and weakens its physical defenses, but can be resisted to completely negate its effects, and has the same casting time as Fireball does - so it's not a primary school for a damage mage, though it could be useful as a secondary school (but still, not for the damage). Additionally, as with Air, Earth Magic damage spells require a minimum of 1 level invested into Earth Magic.

Death Magic damage spells are all single-target effects, now that Contagion is gone, so it's not a very versatile damage school. Not bad as a secondary school for a damage mage, though all of the good damage spells require at least 1 level invested because they are all at Death III or higher (and no, Shadowbolt is not a good damage spell; it's not resistible, but the damage is pitiful and it costs only 6 mana less than a Flame Dart before accounting for the Mage discount; the primary use of Shadowbolt is for reducing spell resistance, not dealing damage).

Life doesn't have any damage spells at all; need I point out how useless that is as a primary school for a damage mage? It's fine as a utility school, but it doesn't really offer anything to a damage mage unless you're fighting a magic-immune target or you only have one available type of elemental damage (you shouldn't, in case that wasn't obvious).

Fire/Water, Fire/Death, and Death/Water are great combinations for damage mages, as they combine a decent mix of AOE and single-target damage spells; additionally, Fire/Water and Fire/Death are slightly superior to Death/Water as Fire needs very little investment in order to gain access to its best damage spells. Since I like to play with tactical damage spellcasters and tactical support spellcasters, I rate the schools with good spells for those types highly - namely, Fire, Water, Air, and Death. I only rated Life so low because, since I usually play with Empires, Life is rarely available enough to be useful to me - it doesn't matter how powerful the school of magic is, if I can't use it it isn't going to rate highly in my ranking of the magic schools; if I usually played Kingdoms rather than Empires, then Death would occupy the position Life does now. Earth ranks just above Life because it's available to me, but it isn't really designed as a primary spellcaster's main school of magic, and since I like my champions to be primary spellcasters, it's mostly fine for me if I have even just one champion in my entire empire that has it, nor am I particularly disappointed by not having it available.

Your argument is that Fire Magic isn't as good a magic school as Air or Water because it doesn't have utility spells like Haste and Slow, which are admittedly powerful, useful spells (and a large part of why Air and Water rank in at #2 and #1 in my listing), but it doesn't speak to the reason why I ranked Fire at #1, that reason being damage. No other school of magic provides as versatile a set of damage spells as Fire does at as little investment, nor is Anointed by Fire completely superior to taking Fire III (for one thing, maintaining that enchantment on a single champion costs as much as one Fireball every 38 turns; for another, it requires you to research Rituals, which isn't a very appealing research line if I don't have a decent source of crystal readily available). Water Magic's only real damage spell is essentially a blunt hammer that works really well on large armies of multi-figure units, but isn't so useful against single-figure units and isn't necessary against small armies. Air Magic's damage spells are either unreliable or put the caster into a bad situation if you want to use the damage, while Earth Magic's damage spells are weak and expensive. Death Magic damage is all single-target, and the good damage spells come no earlier in the tree than Fireball (incidentally, if all you care about is damage and not the mana required to deal it, Fireball is a better single-target damage spell than Drain Life is, and is nearly as good as Touch of Entropy assuming no shard bonuses on either side, though it's much more expensive to cast). Fire gives a flexible set of spells quickly. What more could a damage mage ask of a single school of magic, except for untyped damage on those spells?

What I read from you is all true. But that doesn't really help. Why? I want to answer this by sharing my experience with you.

I now play the second highest difficulty level (I don't know the name in english). I tried several strategies and lost.

Now I try to boost research, population growth and production. The goal is to get an army of mages as soon as possible. To win a battle it is important that your troops can act first (that means they need a high initiative).

- Thus I alwalys take a leader as sovereign. He can greatly raise the initiative of your troops.

- I want my cities reach as soon as possible higher levels. Thus LIFE and EARTH are my favorite magic schools. (A level 4 conclave gives an additional essence and a +2 crystal income.)

If I have luck I get a hero with AIR magic. 25 % experience for my sovereign and up to +5 to initiative for my troops is awesome. AIR is the strongest magic school but as I wrote you first need to be able to produce troops. Then you can bring the focus to strengthen your army.

Until now this is the best way to win at high difficulty level. Let's have an eye to my strategy: If you assume that your enemy traces the same strategy (procucing troops with high initiative) and if you also assume that he has the same requirements (= for example same equipement) as you you will ALWAYS beat his army if you have a leader as sovereign. Why? Because a leader boosts the initiative of your troops. The enemy might have a very strong mage or fighter but his hero will never be able to cast a spell or attack something. If all your troops have a higher iniative then the enemy troops your mages (or archers) will easily be able to kill the enemy hero in the first round. He has no chance to survive and will never be able to do something. If you play with the tarth race this strategy becomes even stronger: Two attacks in the first round greatly decimates the enemy army. In addition to the death enemy hero you will be able to weaken the enemy army so much that the surviving enemy troops are no danger.

PS: My native language is German but I assume you understand me. 

Phew, I am glad there's some discussion about Fire. Because all the consensus in this thread is making my head spin.

Come on people, this is a forum on the Internets! Where's all the flaming and trolling? Geez 

Who said that I only use 3 troops in my army??? The Tarth race is still the strongest one due to the double attack.