"you are so inept, you could f**k up an iron ball with a rubber hammer"

"that baby is so ugly, it's mom hangs a pork chop around it's neck so the dog will play with it."

"did your parents have any children that lived?"

"his family tree does not have any forks in it"

my Uncle's favorite:

"boy, you are so stupid, the best part of you must have become a stain on the mattress."