It's kind of a tricky one. I'm enjoying BF3 almost perversely, it will easily be the game of 2011 I sink the most hours into. But it hasn't moved the medium, to sound all artsy.
Portal 2 was excellent, but I think my choice would be LA Noire. I'm not an emotional guy. I may have been playing the wrong games, but I don't recall ever feeling anything about either pro- or antagonists.
I did in LA Noire. I'm not sure what did it, I'm not sure the amazing animation even had that much to with it... but goddamit Cole, how could you?
In any case (no pun intended), LA Noire would be my pick. Now, I should probably get around to giving it another playthrough.