The one time I played through the game (the PS1 version), I used a knight, a monk and white mage and a black mage. Out of those four, I don't really have a favourite; I found them all to be integral for the entirety of the game. The monk could dish out more damage than the knight as the game progressed, but, if I remember correctly, had weaker defense. Obviously, the black mage was essential against the element-orientated enemies, but I remember Flare not doing a great deal of damage to the final boss - in contrast to the monk and knight's physical attacks, anyway. It was the same for my white mage and Holy, although I highly doubt I would have pulled through the numerous dungeons without her curative spells.

I plan to use the red mage and the thief instead of the white and black mages when I play the PSP version.