The greatest insults are always those meant to insult you, but are actually complimentary, like, "You think you're some kind of smartypants feminist liberal!" (um, yep!), or those so searingly accurate you can't help but admire the quality; only people who know you really well can deliver those, and at their peril. Mr. Shakes can deliver devastatingly accurate blows with a single raised eyebrow and a delicious smirk, which, because I am all too well aware of my own flaws, only makes me laugh my ass off at his wittiness, while lamenting I can hide nothing from him. (The same goes in return, natch.)
I've also gathered quite the collection of hilarious insults from emailers, who thoughtfully take time out of their busy, busy days to tell me what a horrible, stupid, ugly, and/or otherwise objectionable person I am. My recent favorite said, in total:
Yes, yes I am a dumm bith.