My "main" collection is marvel legends and the associated spin off lines. To a lesser degree I get the DC stuff, but I'm more of a Marvel guy at heart. But Batman simply rules, and he has the best rouges gallery in comicdom, so I've got a lot of Batman too.
I try to get the "definitive" versions of each character they put out. Of course with guys like Batman, Spidey and Hulk, that means a lot of figs with similar costumes, just based on different artist's interpretations.
I have a lot of Star Wars stuff going all the way back to '77. I've tapered that off to just getting the occasional cool looking figure. I really have enjoyed the recent McQuarie stuff.
I've been getting select pieces from 25th GI Joe. It is a great collection, but I'm trying hard to avoid figs if I had them as a kid since I still have all my old Joe stuff. Snagged the Tomax & Xamot 2 pack just the other day!!
Man, I should just have work direct deposit a check to Hasbro!
I pick up the occasional anime or video game character too, but that is pretty minor.
The worst thing is that I also make custom figures, so fodder figures of any kind are always a draw. Whatever the collection, if they have interesting parts that I can use for customs I'll get them. Though I try to mostly get that stuff on clearance or in cheap lots from ebay or whatever.