For me, it's Vanquish. Sure, there have been other fantastic games, games with more "impact" if you like. But for me, Vanquish was an important game. Not because it made any contribution to the medium's desperate struggle for viability as an art form, not for its (admittedly terrible) story or acting, but because it exemplifies video games as games. As unadulterated, grab-you-by-the-throat action that tests your reflexes and ability to think under enormous pressure. Where other games told incredible stories or asked the player to confront their own morality, Vanquish distilled the most important part of the medium - gameplay - to its purest and most simple form: perfect controls, excellent level design, and over-the-top outlandish fun.
So I say congratulations to Vanquish. You may not sell as well, but in my opinion you are more important than Black Ops, Fallout: NV, Fable 3, Red Dead and Halo Reach, because you know what you are and what you want to be: a game. And you do it so well.