Author unknown, but I use this one quite often:
"Yeah, I may be fat but you're ugly! I can diet! What can YOU do short of plastic surgery?"
Actually that was Winston Churchill and the real life (and confirmed) story is Bessie Braddock, Labor politician, told him:
"Winston, you are drunk, and what's more, you are disgustingly drunk."
Churchill: "Bessie, my dear, you are ugly, and what's more, you are disgustingly ugly. But tomorrow I shall be sober and you will still be disgustingly ugly."