That's a good question. I know I've missed out on a lot of good games this year, ones I will try to catch up on this summer (Fable 2, Fallout 3, Prince of Persia, Left 4 Dead), so of course my GOTY may not be the same GOTY I would pick had I enough time to try every game. All things considered, though, I'm pretty happy with my choice.
I've gotta give it to Braid. XBLA game or not, it was by far my favorite experience of the year. I loved the storytelling, the mechanics were solid, the art direction was strong, but what really pushed the game through the roof for me was the way they all came together. the game mechanics echoed the man's desire to go back in time, which in turn was echoed in the rewind time mechanic; platforming, which helped the game establish itself as a kind of parody of Mario, took many of the conventions and turned them completely upside-down (especially the ending). Even the secret stars gave a lot to think about.
Honorable mentions include The World Ends With You (fun game with a great soundtrack); Gears of War 2 (I loved the first game and the campaign really exceeded my expectations, but matchmaking/ranking problems, the inability to play co-op with a guest, and planting grenades) took away from the overall experience in ways I really wish it hadn't; LittleBigPlanet, which has an awesome art design and fun levels, but I haven't had enough time to scratch the surface of the game and utilize its creation tools.