In terms of sheer addictiveness and replay, I'd have to go with Black Ops Multiplayer. Def. fun to play no matter what mood you're in or how long it's been. Not my favorite game by any stretch, but def. one hell of a value, esp. on PSN b/c there's no annual fee.
Would've been ME2 for me, but since I got on PS3, it wouldn't apply to 2010.