Yithril because juggernauts.

Played them all and i must say pariden. but nowadaays scouts custom is always what i play. mayhaps i try them all on insane butbut no scouts i kinda didnt feel like it. my first dig scrapped cus boredom things happend fast and i was apparently nowhere nere them. ill get around to them in time now that the game is final the first im going to play is the scouts or the mancers 3move is pretty pow ah custom mancer yeah thats what im gonna do.

edit airshard ini pariden thats why.

i think i played at least one full campaign with each of them (though some  of that was in FE, so the rules have changed quite a bit and they may be a bit different now)

that's a really tough decision. Pariden is definitely among my favorites, but i also enjoy Capitar and Kraxis a lot. Gilden and Yithril aren't too bad either, though a bit one dimensional.

EDIT: totally skipped the "and why?" part of the question

Pariden is great because all their abilities work together - scrying pools give you more mana when you need it, Monoliths allow you to expand influence quickly and grab shards and the shards power up the stats of your troops and also improve many spells in the multiple spellbooks you can have on every hero. their default sovereign, Procipinee, starts with the 3 (IMO) best spell early game spell schools and she can be tweaked to be a summoner, a great support spellcaster (heal/buff/beduff) or a damage specced evoker (storm & blizzard are pretty solid; and you can always just slap on Annointed by Fire or buy a fire epllbook later if you want to go that toute).

very nice setup IMO.

capitar is not as nice, but the +1 move and road building make their units very mobile, the accuracy buff gives them a nice damage boost, their orsesa re a bit better than default and their sovereign is probably the best trait combination you could come up with for an epic commander. he can grab battle cry at level 5, then pick up life and air spells to complement his arsenal (heal/AoE heal/growth/haste/titans breath/cloudwalk/tornado/guardian wind - all of them are great utility skills for a commander)

Kraxis is a bit odd. actually i mostly like their troops (spear&shield + the tactical fortifications and the krax blood make them very tough to beat, even with crappy early tech items). their sov isn't that great. wealthy is nice and water is a nice early on, fire is more usefaul later on. can be a pretty decent damage caster if specced for it (fireball, flamedart & blizzard). never got much use for the whole Betrayer faction trait or the silver tongue sov ability, though.

Gilden and Yithril are easy to understand and don't offer a lot of finesse, but their traits are solid and support what they are doing. Gilden is all about having impenetrable high armor units and Yithril is all about brining the pain with their super axes wielded by super soldiers.

to be honest, i think i like all of the premade factions. if you think about the traits a bit and form a strategy around their abilites, they can all work out nicely.

Dude that is a hard question for me to answer... I play a lot of standard factions because I'm trying to help create better customized units for each faction to play against in future games.  I can tell you what my least favorites are:  Tarth (weak) and Gilden (boring) and Kraxis (just okay) and Umber (well, beastlord is cool!).  Hmmmm... let me think...

Altar:  Love the henchmen and Rush ability

Pariden:  Love Summoning, AM, and the Crown

Capitar:  Love the extra movement

Yithril:  Love Juggs

Magnar:  Love slaves and Fire Tongues

Resoln:  Love Summoning and lots of Death shards

So the answer is... if I'm playing to win, I would say Altar.  If I'm playing for fun, it would be Yithril, Magnar, or Resoln.

Paradin ... because magic rules. Scrying Pools and Arcane Monoliths are OP.

Ouch urux is so Ouch

Resoln. Corrupt every shard -> spam wither.

I just watched your LP last night (you kept me up until 2am..I work at 630!) and as a new player, I realized I had played Resoln wrong the first time. Granted, that was my first game and I played on Normal and won easily, but I didn't even use Corruption on any of my shards, went up the Evoker and Spell Mastery lines, and used her Dirge of Ceresa ability to start with wither and graveseal on the big baddies. While the Dirge ticked off every turn my tanks held them back and I would use spears as supplemental damage and instant fireballs to nuke grouped up enemies.

I also did not consider how the shadow warg could be so powerful early. I used the familiar to support cast graveseals and occasionally flame darts. 

Now I want to go back and play her again on Hard or Expert level to see what I can do using Corruption and a beefed up Wither rather than trying to maximize Dirge damage and sustain through fights.

I mostly only play Custom Factions, but I've really grown to like the Trog and revamped Berserk ability.  If my troops are built right, they get buffed, kill stuff, and I don't have to do as much pointing & clicking.

I have to say my favorite thing about LH is how much thought this question requires.  Each faction has such fun and different flavors and abilities that choosing just one is rather difficult.  

Pariden's spellbooks, scrying pools and arcane monoliths are very effective, and very fun.  So are Yithril's Juggs, in a very different way.  The Altar rush ability is freaking huge in tactical combat, and the quest maps allow for a great hero-based strategy.

But then you have the Ironeer's unmatched defense, allowing for a nigh-unbreakable front line in tactical combat.  And Magnar's no-wage, disposable slave armies.  

I'm frankly still figuring out the right approach for some of the factions, but I'm sure as hell looking forward to the effort.