AvP2 on PC all the way!
I've played just about all of the games besides the ones on Genesis and Gameboy... I enjoy them all, but AvP2 is one of the best games of all time in my book.
The Alien and Predator games, individually, aren't that interesting to me... I hope Colonial Marines changes that. But I do enjoy Predator: Concrete Jungle for the sheer fact that it's easy as hell (even on the hardest setting) and you can just whip through it. Makes you feel good, like a skilled hunter. (As opposed to Predator on NES... which makes you want to throw the controller through the tv because Arnie doesn't know how to jump correctly)