I have played in a lot of games where I had moments when I laughed to the point I couldn't talk. What's more, they were rarely "comedy" games like Toon where the humor seems so contrived. But when I really think about it, the funniest game I have played was honestly my "Ultimate Posse" campaign using the Marvel Heroic RPG.
Ben's superhero "Union Galactic" was played for comic relief in a way that was so consistent and yet not hokey or annoying, complete with accent. Couple him with the Ferret, who sort of bumbled his way through superhero-dom with that sincerity that showed he was trying, in the words of Vonnegut, to bargain with his destiny in good faith. For my part I had Mrs. Robot, the old-lady's brain in a robot body who seemed completely comfortable with it ("I'm making the best of a bad situation.") Samhkara was the straight man (woman) of the group, perpetually grinding her imaginary teeth at the antics of her teammates to the point where the others would actually provoke her. But when she was captured once, the group was all business about getting her back. What really make it all work was that all the PC's seemed to like each other, just as all the players did.
For all my bitching about MHR on this blog, it was a damn fine campaign.