This is a question I've always wondered about. In many of the later games the answer is more clearcut, as there are characters that can just plan do more damage, or characters are so cookie-cutter in battle that it doesn't matter. Each character in FFIX has his or her own specific strengths and weaknesses. Personally I used Garnet, Vivi, Zidane, and Steiner. I feel like optimally, Garnet should be switched out for either Quina for auto-life, or for Eiko's dual white magic and Phoenix revive ability.
EDIT: Also did anyone ever smurfing use Amarant? I've always wondered if he was good but was too lazy to find out.