Hangar Defenses are always good, especially if you are Advent, they have an awesome shield boost research that makes it really really annoying for your opponents to kill your nearby tactical structures. Repair bays are quite useful as well, as they can help keep your other defenses alive.
Defense plaforms (IE Gauss) aren't really so great, but you should probably still build a few of them, though hangars/repair bays should probably be your top priority.
Make sure to support your SB with repair bays if possible, and if you are advent surround them with hangar defenses too, cause it makes your star base even more annoying to destroy than it already is.
The TEC Hangar Defense gets several powerful, though short range, autocannons through research that lets it tear up most things that aren't capital ships, and it likes to eat strike craft, unless there's a whole lot of them.
The Vasari Hangar Defense abil is probably the best though, you can just freeze a bunch of strike craft in place, letting your own mop them up.
Starbases are, obviously, your most powerful defensive option. For the TEC, the must have is pretty much going to be the "big red button" as it is known, or Last Resort, against an opponent who isn't being cautious, this can annihilate an entire fleet (make sure to get rank 2 as it dramatically increases the damage), and unless you are totally ultra mega screwed (I.E, your opponent has like 300 bombers coming at your SB), you can allow your Argonev to wear down the enemy fleet first, then use Last Resort, possibly resulting in a lot more ships destroyed.
For the Advent, you should probably make sure to get Meteor Storm, as this is a powerful AoE which can quickly destroy most frigate fleets, and help a good bit with wearing down larger ships. (unsure if rank 1 is sufficient or not, someone more knowledgable like Goa can probably tell you). As mentioned earlier your Hangar Defenses should surround the SB, these can help keep bombers off of it, as well as help keep it alive long enough for Meteor Storm to do lots of damage.
As for the Vasari, the Orkulus is probably the most versatile starbase in the game. Though it lacks the uncapped AoEs of the Transcenia and Argonev, it makes up for this with blistering firepower, hull strength that can make a Titan jealous, and some of the best armor in the game (though this is mostly only true if you go Vasari Rebels), not to mention it is the only starbase that is capable of movement (however slow it may be, it can still do it). It can also be an extremely overpowered offensive weapon as Vasari Rebels get starbase mobilization, which lets their Orkulus (orkies) phase jump. But please, don't use this, that's for lame people 
. Frontal deflector shield is a good thing to have, as is debris vortex, as this will help offset the fact that the Orkulus repairs reaalllyy reallly slowwwlyyyyyy. You will probably only need these upgrades in high traffic areas though, as generally an orky without these will be able to handle most smaller attacks, in a huge battle though these will allow an Orkulus to take a ridiculous amount of punishment and keep fighting.
