If you really want to roll to see how well the character does, as opposed to how verbose the player is, I'd use, well, I don't know.
Depends on what sort of insult it would be.
Innuendo - for the sly underhand suggestion that might pass over the opponent's head, but leave the bystanders chuckling.
Perform - for the exact right comment at the exact right time. Would also let the character know more entertaining insults.
Intimidate - perhaps, to provoke a fight.
Bluff - ah, for the outright slander.
etc, etc.
But most of it should be left to pure roleplaying.
(Robin Laws had an article in a recent Dragon (295?, 294?) which might help.)