Super Mario Galaxy 2
No part of this should be surprising. Galaxy 2 is basically like the original, only improved in basically every way. Not only is it a game that I never stop smiling as I play, it's a game that had The Gotta, the rarest and most desirable of all entertainment qualities: you gotta keep going, gotta see the next level, gotta get at least one more star. It wasn't just excellent, it was addictive.
Excellence was the primary defining characteristic, though. Everything about it oozes quality like few other games in the industry, a result of craft so long-practiced and loving that it is not directly comparable to any other level of craft in the medium.
Ah, it makes me so happy to play.
Super Mario Galaxy 2 is - I'm willing to say as of now - the best game released in 2010.