Quina, Steiner, Dagger and Eiko.
For starters, everybody is capable of dealing 9999 damage plus:
Quina: Awesome and very helpful blue magic including frog drop, mighty guard, angel's snack and auto-life. Plus he is the coolest character.
Steiner: Very powerful sword attacks, plus I liked his character and also his music.
Eiko and Dagger: Both use powerful summons and white magic.