1. I like all the turrets, Because I like to have a large number of ranges/reload times/ damage types. But my favorite, i'd have to say, is the level three MLRS. Spamming these will eradicate fleets, murder starships, and effectively cover two wormholes on a planet you don't want to take but have to travel through.
2. I dislike lightning turrets. They don't seem to do a hell of a lot of damage and, worse, you only get 33 of them. Thinking about them again, I could probably work them into most of my defenses, but I just found 33 distasteful.
3. I HAVE to have, on every planet, spider turrets. Anyone wondering what's the point think of it this way: You have a planet with five or so enemy entry points. This planet is important enough you want to defend it heavily. You can't afford/don't have the number of turrets to defend every point. This is what I do:
I put tractor threes on each wormhole, maybe two or three. Then I build five-ten spider turrets + 5-10 snipers. Finally I heavily defend just around my command station. Now, the enemy units will get stuck by the tractors and the spiders will pick off their engines one by one. Even if they're core ships they wont be able to get anywhere near my command station. Depending on how far the station is, you can take out 100-200 engines like this. And then just have the spider turrets slowly kill them. Starships can go down like this too. The only thing you have to worry about are melee ships and inf engine ships.
4 My special turret are counter turrets. I played a game where the AI just spammed missle frigs so I put a few counters down and problem solved. Worked the same way against dark-matter firing parylyzer.