I like ho this thread opens the door for anyone else to do other threads such as Best Game of 1990 - 2000, 1980 - 1990, etc...
Anyway, to answer the thread proper...Well I guess I shouldn't yet since 2011 isn't over yet. Tell you what I will do though, I'll tweak the timeline to 2000 - 2010 and pick on of those games, kay?
My Game of the Decade is going to be 2004's Grand Theft Auto: San Andreas. It did everything the original Saint's Row did plus a little bit more (like being able to lock onto targets and one-shot kill cars). I also loved the RPG features, which allowed you to play as Action Hank one minute then turn CJ into The Notorious B.I.G. The missions were a bit tougher than Vice City and 3 but they were all doable and once done you feel pretty accomplished. The only criticism is how driving-heavy the game is, with a ton of races that you really can't skip. I've also had a lot of trouble getting my driving stats to increase but that's probably just me.
Runners up and, notable games of the last decade include the Max Payne series, Pokemon series, Castlevania: Aria of Sorrow and DS, Obscure, Call of Cthulhu: Dark Corners of the Earth, Guilty Gear X, Bully, Dragon Ball Z Tenkaichi Budakai 3, Saint's Row and 2 and, many MANY others.