The funniest song I have ever heard is "U Mean I'm Not" by Black Sheep, off of their album A Wolf in Sheep's Clothing
, which is a rap album from 1991 that is widely considered one of the greatest ever. I am not at all a rap fan, but this song is a brilliant, brilliant satire of the kind of stereotypical rap I hate. It's just a one and a half minute rush of exaggerated swagger that applies more today than it did when it was released. The first time I heard it, I couldn't stop laughing.
I'm not going to link it though... it's extremely vulgar.
Just look it up if you're curious.