Well... It depends entirely on what you look for in a game. Those games are pretty different.
If I had to put my money on any of those games being super-mega-awesome it'd be Mass Effect 2, closely followed by Assassin's Creed 2. But, again, it depends on what you want.
I also have to say that the new Splinter Cell looks damn good - but appart from that I'm not personally looking forward to Bioshock 2 or ODST. And I have no idea what Alpha Protocol is.
Last time I checked, Bioshock 2, Splinter Cell Conviction and Mass Effect 2 weren't being released until next year.