Fighter - Fighter is the MVP of any party, given his high ATK and DEF. In most fights, he will have both dealt and absorbed the most damage.

Red mage - He's the king of utility, having access to both white and black magic and therefore having the buffs from both schools. He's also got a decent ATK.

White mage - Going without the white mage would make for a very tough playthrough. Her healing magic is virtually essential. Every once in awhile she does some acceptable damage with her hammer (critical hit!) - well, at least more than the BM, until he gets cat claws anyway.

Black mage - You'll be glad you have him around when you accidentally step back on that panel in front of that treasure chest and you're having to fight that difficult encounter again. Those encounters placed in front of the treasure chests need to be handled with care and the black mage's spells can help you be done with them quickly before your party members take too much damage. There's also nuking bosses. Oh, and he's good for taking out large enemy groups with his AOE spells, too.

Black belt - At least he hits hard and is cheap to outfit.

Thief - Maybe he wouldn't suck if he actually did what he was supposed to... if he actually successfully dodged a lot of enemy attacks and had his attacks land more often than the Fighter's. Truth is, I've seen him miss too many times and I've rarely seen him dodge an enemy's attack. Throw those things in with the fact that he's squishy and has kinda crappy ATK, and you've got a true loser.