This is one area-"insults"-that I rarely have a list of "comebacks" for, probably because I am extremely careful not to provoke, offend, or insult others.
There has been, however, the occasional uncomfortable moment when an individual has hurled an insulting remark at me. Over the passage of a great deal of time, the one comeback I latched onto that works for me and which I feel comfortable saying, during the very rare occasion when someone hurls an insult at me, is, "Bite me!" and you have to say that firmly (no shouting), and with a steady, penetrating, hard gaze, focused directly into the eyes of the person insulting you with his/her remark(s). No hand-waving and no gestures, just that penetrating stare and firm voice, showing you aren't moveable and you don't crumble.
I haven't had to use that phrase very often, probably because I am not easily offended and I tend to be mindful and considerate of others. When I have chosen to make that statement firmly, there suddenly occurs, this "pause," a palpable moment of silence, except maybe, for the heavy breathing of the other person, waiting to pounce on you again with something more. Yet, it never happens, not yet. Each time, the person just sort of "backs off," sometimes "away," then returns in a more amiable state, and sometimes with an apology.
"Bite me" hasn't been around in use all that long, I don't think. From that, you can probably figure that I used to just bear the brunt of the remark, and perhaps, I left, rather than the offender. It's certainly easy to remember, which is good, considering that I am now having difficulty with that.