Here I will share some of my most cherished insults with the masses (or at least the three or so people that will probably read this)
"I hope you die while masterbating and your mother finds your body" This is instantly recognizable as a heinous insult. And to wish such a thing on someone's mother is as wrong as it is hilarious.
"Your breath smells like the inside of a prosthetic leg" I like this one because it combines the disgust of imagining the stench with the discomfort of ridiculing the disabled.
"Your mother is such a frail prostitute that she only survives the harsh winters on the street because of the insulative properties of her ample beard" I just made this one up. Will try it out on a close friend soon.
"Your personal hygene could scare a ten pound rat off a New York city garbage barge." This one is slightly modified from an original by the great Doug Stanhope.
I will try to remember and craft a few more. But first, research. I need to look up obscure and crippling condtions to mock, and find an urban thesaurus to discover more variations of the word "retard".