Quake Live - most other shooters are way to complex for me. If I play a shooter I just want to frag a few guys. Starcraft and SupCom consume all my need for games where I have to really thing while playing I guess. Being able to play via Browser is really nice for casual gaming too. sure you did - but now I am here
I have to pick Halo 3 because I haven't really played the others, the only other on the list that I played is a bit of Left 4 Dead, but not really that much. I do have to admit to playing Doom64 for many many many hours, that and the original Turok, going back again to my N64 years ago which has left me with fond memories (and I still have)
Big fan of Halo 3 as it is the game I have the most fun with when I'm playing with my friends. Nothing quite like having a nice little game with your best buddies in front of the same big screen TV playing against a bunch of idiots and slaughtering them and laughing about it afterwards :lol:
TF2 followed by COD4 followed by L4D. They are all really top quality games but I always keep being pulled back into TF2. It is the best MP game I have played and the one that I have played the most (Over 150 hours).