I played a lot of games that I liked this year but if I were picking one I think I'd go with Red Dead Redemption. Mostly for the originality, there have been western games before and there have been GTA sandbox games before but this game felt unique, had a ton of content packed into it, told a good story both in the writing and voice acting, and was just something I really enjoyed playing through.

Honorable Mentions
Mass Effect 2 - The game I almost chose for my GOTY. Loved the story and all the improvements over ME1
Battlefield: Bad Company 2 - Great singleplayer and multiplayer
Halo: Reach - The Halo series has always been pretty good but never my favorite series. This one really stands above the other Halo games
Transformers: War for Cybertron - Got a lot more enjoyment out of this game than I was expecting, great use of the license and really fun co-op campaign
Call of Duty: Black Ops - Great multiplayer, made all the right improvements on MW2
World of Warcraft: Cataclysm - Blizzard made heroics feel heroic again, the new damage/healing model makes tanking more fun, worgen are great