I usually go with my Zidane, Vivi, Eiko and Steiner.
Steiner - Shock is awesome, 9999 damage, his break attack work well, Climhazzard inflict 9000+ damage to all, I don't really care his trances.
Vivi - Doomsday heal me and attack enemies (I equip Shadow-absorbing armor which heal me instead of attacking me, I use it against Ozma, sometimes Ozma absorb it but rarely in my data). Flare is just awesome against one-boss-battle, I equip ReflectX2 which double Vivi's magic when it was reflected and it make Vivi magic goes 9999. Double Black is superb.
Eiko - Being able to learn Curaga and other white magic fast than Garnet, Eiko seriously is better than Garnet, Eiko also specialize in the white magic: Haste, Full-Life, Dispel, Might, Jewel, Holy. Holy can inflict 9999 damage sometimes against Shadow enemy. Moreover, her Curaga heal a lot of my party (I use Curaga on my party in one turn). Haste strengthen Steiner speed, Might increase Zidane's attack, Jewel is like Drain, Dispel remove enemy positive status and Full-Life REVIVES THE MEMBER, Double-White can revive two character in full-HP! Seriously, Eiko is better than Garnet, she also know Phoenix, your lifesaver.
Zidane - STEAL! There's no other better thief than Zidane, Theivery rack-up much damage and his Grand Lethal is amazing.