I'd pick Esuna without a second thought. I mean come on, it cures Blindness, Silence, Confusion, Berserk, Poison, the list goes on. Hell in FFV it even cures Old! Now wouldn't that be crazy?

Honorable mention to Resist which prevents such ailments pre-emptively, but since it also prevents curing them if you unwittingly already have them it's a bit too much of a double-edged sword for me.