I often have a hilarious time playing RPGs, and the many games tend to run together in recollection, so picking one single “funniest game” is pretty much impossible. I can tell you the funniest recent game I was in: I played a game of Little Wizards at Gen Con this year. It was quite fun and we broke the GM at least twice, though I don’t recall the particular parts that were so funny. I think one of them was the 11-year-old complaining that we couldn’t use the fountain in the town square to brew a giant draught of tea to cure the town of fairy dust-laden chocolates because the water is recirculated and thus not sanitary, to which someone quipped “we’re responsible for preventing fairy dust, not dysentery”.
Another funny convention game I can heartily recommend is Advanced Dimensional Green Ninja-Educational Preparatory Super-Elementary Fortress 555, when run by Akira and/or Jason. I had a hand in it’s origin, though it evolved quite a bit before it was published, but it really is those two that bring it to life. I can put funny stuff on paper, and even play a funny character, but running a funny game is something Akira, in particular, excels at.
But the funniest convention games I can (vaguely) recall were all QAGS games, run by the Hex Games crew. It’s been enough years that I can’t recall any details, much less which games were which, but I can remember some of the settings/genres: classic Hammer Films-style monsters as ‘80s buddy cops; “laser ponies”; swinging ‘60s superspies with superpowers; Discworld-ish deconstructed fantasy. All were hilarious, and while QAGS can be used for serious games and the Hex Games folks sometimes do so, if you ever get a chance to play one of their wacky games, do it! They’ve been reliably the most fun I’ve had playing convention games, often necessitating breaks to recover from the laughter.