Warrior/Knight- Pwns it from beginning to end.
Monk/Master- Best Physical attacker from about Lvl. 10>.
White Mage/Wizard- Exit & Life Spells. Good against undead. In the original a sucky attack.
Black Mage/Wizard- Good assortment of spells. Ok attack w/ Cat Claw. In original many spells bugged.
Red Mage/Wizard- Overtly Balanced Character. Doesn't stand out after Class Change, as he doesn't receive any really good spells.
Theif/Ninja- Opposite of Red. Kind of useless until Class Change. High Evasion Rate & Escape Rate bugged on the NES version, making him a poor man's Warrior. Doesn't steal.