Currently the only game I'm playing is Left4Dead which is one of the best zombie games ever made. All I can say is 4 player co-op fighting through hundreds of zombies is good times, especially with your friends.
I played through the Gears Of War 2 campaign and was generally entertained. Don't expect to be amazed by the story or dialogue, but do expect to feel extremely badass.
As for Bioshock, it is one of my all time favorites. The story and ambience of the game is simply amazing. The gameplay does not disappoint either. Top quality game that for 15 dollars, is truly worth it.
Cod4 is a great game as well, and the multiplayer keeps its replay value extremely high.
Diablo 3 looks pretty good, although to me it seems like alot more of the same, which isn't necessarily a bad thing.