I think it's near impossible for me to choose just 1, so I'll split it into different cateagories:
Graphics: Crysis, just WOW
Characters: Mass Effect 2, such emphasis on character development. Made a nice change.
Polish & technical acheivement: Uncharted 2, mixed so many play styles so well and told a great story at the sametime.
Storytelling: Half-Life 2 + episodes, in particular the end of Ep2.
Sandbox/Content: Oblivion