Depends on what criteria we're judging this by.
New IP that pleasantly surprised and shocked many gamers? In a toss-up between Spec Ops and Dishonored, I'd pick the latter. Both great games that shook up the FPS/TPS formats just a bit (Dishonored being Deus Ex meets Thief helped too).
A total game-changer for its respective genre? Guild Wars 2 has it all. I'd be surprised if the industry doesn't copy its model in the years moving forward. It's that good.
A sequel that refines everything that its predecessor did well? GW2 and Borderlands 2 both did it incredibly well.
Hell, RE6 could be a dark horse contender once people realize how much content the game is stacked with.
I don't consider X-COM, Diablo III or ME3 (or its fanboys) in my list, as they were good, but underwhelming in at least one way, shape or form (X-COM for not bringing back the horror and flexibility of the original games, D3 for its grindfest always-online gameplay, and ME3 for just about everything besides the combat). I don't see them winning anything this year.
I also don't see Halo 4 making much impact this year. Much shorter than previous campaigns, and mainly exists as a set-up for further games. Far Cry 3 could be awesome, but I haven't seen much gameplay to make an informed opinion.