In no particular order....
1) Bioshock - Completely immersive game that hooked me from start to finish. Thanks to the Plasmids and Tonics there are many ways you can approach any fight, and the world of Rapture was completely believable and just sucked you in. I completed it in a weekend as I simply couldn't put it down.
2) Call of Duty 4 - brilliant single player experience, but easily the multiplayer game of the year in my eyes. The perk system is great and a wonderful way to customise the experience to your own liking. I'll still be playing this online in 6 months easily, a sign of a truly great MP game.
3) Mass Effect - not completed it, but so far it's an amazing game. Incredible detail, voince acting and visuals combine to make this a must play game, whether you're an RPG fan or not. The fact the combat is real time makes it feel less RPG than action game, a welcome change from the turn based combat we usually get.
If I had to pick one, Bioshock would get it currently, but that may well change by the time I've finished Mass Effect.