I think we should become better strangers. (An old Shakespere term for fuck off)
As for my own favourites.
"Your existence, it pisses me off."
"Your birth certificate is an apology from the condom factory."
"You must be the arithmetic man you add trouble, subtract pleasure, divide attention and multiply ignorance."
"Do you have to leave so soon, I was just about to poison your drink."
I've never had the chance to use any of these, except for the first one but by god I want to. To someone who deserves it of course.
Also speaking of insults, Winston Churchill was a king on the subject.
Woman: "Winston you're drunk"
Winston: "Maybe so madam, but tommorow I will be sober and you will still be ugly."
Woman: "If I were your wife, I would poison your tea."
Winston: "Madam, if I were your husband I would drink it."