I always prefer to customize my insults to ensure a deeper “twisted knife“ wound. But if I had to pick one thing from over the years that stuck with me was “i bet you still kiss your dad on the lips”
i just improvise on the go, tbh. my insults are just roasting someone based on the situatuons.
but i love insulting someone as "clown", theres nothing worse. or i say "you're mental" (it sounds better in spanish). i love saying bastard, too.