These are the games that made me laugh the hardest:
Portal. If you like dark humor and hilarious dialog by a deadpan computer, you will absolutely LOVE this game!
Paper Mario: The Thousand Year Door. If you want a game with Mel Brooks style humor, except clean, this is the game for you. Seriously, every chance they get, they put in a joke that you probably should have seen coming, but didn't.
Mario and Luigi (series). If you want a Mario RPG game that isn't Paper Mario, check these out. The humor mostly revolves around having lighthearted moments to balance out the more serious moments, but there's still some pretty funny stuff.
Earthbound. The turn-based combat in this RPG is generic at best, but what kept me playing it was the random humor. It has enemies such as the New Age Retro Hippie and the Annoying Old Party Man, a story scenario with a cult that wants everything in the world to be painted blue, moving to new areas by using a "pencil eraser" to remove giant pencils in the middle of the road, and a boss named Master Belch, who is basically a pile of puke with a face. The status effects are equally humorous. There's Hunger, which is cured by eating something, Homesickness, which is cured by calling your mom on the phone, and Mushroom Head, which makes a mushroom grow on your head that messes with the controls and is cured by having a doctor surgically remove the mushroom. Don't play unless you LOVE random weird humor, though. That's the only kind of humor this game has.
I haven't played Psychonauts, so I can't accurately judge that one.
