The absolute funniest is a (white) trash game called Country Justice: Revenge Of The Rednecks (?) which actually tries to be funny, but then it's so hilariously bad you'll laugh your head off in the first 5 minutes. That game is un-fucking-believable, I seriously don't understand how Big Rigs got so popular and this one not.
Anyway, Dungeons of Dredmor is pretty histerical too. One day my game crashed on start, and then an achievement popped up saying "Suddenly, the dungeon collapses!" Man I fell down from my chair laughing at that; probably the happiest game crash of my life. Then, there are some funny ingame elements such as "Congratulations! You died!" which never gets old, and an enemy character simply named "Unfriendly AI" which looks like a Protectron from the Fallout games... sorta.
Other funny games include Wolfschanze (same reasons as Country Justice), Sniper (or Snajper, some european game), Eat Lead: The Return Of Matt Hazard ("I'll soak your ass!") and Super Meat Boy for its cutscenes.