I have a few. My favorite was invented by a friend when she just called a fellow "Single-helix mouth-breather"
I have used "Limp-dick dog-fucker" when suffering from road-rage.
"May you suffer an unfortunate incident involving a barbed dildo," is another old favorite.
When younger, I used to use "Go get a blowjob from a lawnmower!"
But currently, my favorite is, "Fuck you, the horse you rode in one, the blacksmith that shoe-ed it, and his entire goddamned family!"
The only one I regret was telling someone who had angered me almost to the point of violence was, "You are a mistake of nature I can correct."