I know you are but what am I?
The most offensive comeback in the history of being offensive. It makes 'your mom' look like a compliment.
He said I was a piece of trash. I said, 'I know you are, but what am I?' and he got a snack.
She said I was a joke. I said, 'I know you are, but what am I?' and she died.
He said I was a disaster. I said, 'I know you are, but what am I?' and he left.