It's a tie between Bioshock 2 and Phoenix Wright: Ace Attorny, Trials and Tribulations
I knew Bioshock 2 would be fun, but DAMN. The online multiplayer being fun? I hadn't expected that. And you got a FUCKING HARPOON GUN AND DRILL. I'm sorry, but killing people with either of those things is a recipe for fun. Especially when your drill can reflect bullets back at people.
Trials and Tribulations took what the previous two games had and shot it up by a ton. The story was EPIC. Godot was EPIC. The writing was AMAZING. And at one point, you play as Edgeworth, as a defense attorny, against Von Karma as the prosecution. FUCKING YES.