The most devastating insults are often surprising, creative, and skewer our self-esteem in ways we couldn't predict.
Having someone call you "stupid" or "annoying" or "ugly" with no flourish is much easier to brush off, but having someone whip out their Masters in Creative Writing just to roast you is a vastly different experience. And as with all dynamics of dishing and taking, it's far more fun when you're the one doling out the insult.
"You've gotta stop using your head as just a container for your teeth."
"You look like the kind of person who could fail a DNA test."
I held the door open for an old Black woman at work one day (I am pasty white) and she looked at me dead in the eye and said "F*ck you mayonnaise monkey" and went through the door like I was not there.