I think men could answer this better than women, because they know what would insult them most (unless the women know specifically, like @JLeslie‘s answer).
From the guys I know, I am guessing “homo” or “pussy.”
In the thread that Adirondackwannabe asked, someone referred to the word “cunt” as a euphemism. You could say “pussy” is a euphemism, but saying to a man “you’re a real pussy!” is not something most men would appreciate.
Motherfucker is also probably a bad one, too. I think a man could answer better what insults them the most.