Portal 2 - Not as perfectly crafted as Portal 1, but offers so much more in terms of gameplay.
Batman Arkham Asylum - As soon as I started playing this game I knew it was going to be fantastic. An amazing all round experience. Let's hope Arkham City is this good.
Fallout 3 - I don't think I've ever been so lost in a game world as I was in the Capital Wasteland. Oblivion was a close contender for this spot, but Fallout got to me first.
Final Fantasy X - Probably still my favourite game of all time. I've played this game over and over and never got tired of the plot, setting or characters, not to mention the best battle and leveling systems of any FF IMO.
Sonic the Hedgehog 3 (and Knuckles, because that's how the game was meant to be played) - Token nostalgia item, plus the best 2D sonic game, the series that made me the gamer I am today. I'll always love this game.
So many others I could have added to this list, it almost makes me sad to leave them off. I know loads of them will get shout outs from everyone else though, so that makes me feel a little better.