Took me a minute to think of this one, and I'm still not sure of my answer. But I'd have to say it's a toss up between Portal and No One Lives Forever. In Portal's case, the humor was very dark and left me feeling vaguely horrified, but it was still amusing. NOLF's humor was more upfront, and completely absurd, though lovably so. I mean, what's not perfect about fighting to stop a criminal organization that uses a puppet to deliver its threats and kills people via farting? That game had me cracking up at every turn.

I may have to add Persona 4 to this list, too. I've just started playing it, and while I'm not laughing with it very much, I am definitely laughing at it a lot. Mostly because I keep thinking. "Dude, Jeff and Vinny are going to love this part when they get to it on the Endurance Run"