I've used my fair share of everyone *except* Eiko. As far as I'm concerned, there's not a whole lot that's special about her.

On my last play through I used Zidane, Amarant, Freya, and Steiner most often. On this most recent playthrough, I'm putting the effort into maximizing the damage output of a Zidane, Quina, Freya, Steiner party. =P