The Fuddruckers gag
There's something about this gag with the Fuddruckers evolution. Even if it's your first time watching the movie, you immediately know where the joke is going, and you know exactly how it will conclude. Yet, it's still laugh-out-loud funny when you see the final form of BUTTFUCKERS.
Why, why is it so funny? I can't explain it other than to say it appeals to the perverted teenage boy in me somehow. Kind of like laughing at farts I guess.