From my personal repertoire:
Yo' momma's so nice, she'd give me the hair off her back!
Yo' momma's like a fridge..people always stickin' their meat in her..
Yo' momma's like a bus - guys gettin' on and off of her all day long..
Yo' momma's so fat, she went to a party in a red dress, people screamed "KOOL-AID!!"
And one my friend made up, I still don't get it to this day, but still my fave:
Yo' momma's like a bus... she moves.