Predator and the Alien are actually tied for me.
I absolutely LOVE playing as both harbingers of death from two separate franchises.
The Alien is the fast, three dimensional, psychonecrosexual, cosmic parasite that is the most rewarding to play as, especially, ESPECIALLY, when you destroy disc spamming Predators, tail raping them from behind and biting off their faces when they least expect it.
The Predator. Hunter, warrior and Dutch's worst nightmare. Its weapons and gadgets, coupled by that feeling that you just seem so superior in every way, yet a single mistake could cost you your life. I love it, the thrill of the hunt. Stalking human prey, when you don't know when you're being stalked by ANOTHER nightmare. Yet, I've come to despise disc spamming Predators, which is really, really sad. They could be so much more...foolish young unbloodeds...
The Marine, my second favorite. The underdog, the explosive lead spraying death machine. The "ultimate" badass. Oh, how I agree with Hudson now that I actually stepped into the shoes of the Marine. They may be the weakest in terms of health and melee, but with the right skills, you can OWN the battlefield through sheer, unmatched stopping and firepower. It feels so BADASS to take down hordes of Aliens with the shotty, killing a Pred point-blank with the iconic pulse rifle while constantly bobbing your head to that beautiful tune your motion tracker produces.
Yes, I love this game.