All depends on whats important to them. If like the above, they value their intelligence, they hate it most when you insult that (stupid, ignorant, naive etc...). If they value being non promiscuous, they'll hate it when you insult that (slut, whore, cum dumpster etc...). And if they're self conscious about anything, they'll hate that the most (fat, hooked nose, cottage cheese thighs). I've found cunt to be the best general insult, may not hurt them the most, but it'll do damage regardless.
Or you can go for the ice pick and stick with stupid slutty cunt. Then again, my last girlfriend really hated it when I called her Queen of the Harpies, usually because I'd use it just to belittle whatever arguement she was picking, but frankly say anything to her enough times during those arguements and she'll hate it. I could have called her a sour puss puss and she'd probably still come at me with the fork.