If I had to rank(based on NES version)
1) Fighter/Knight- expensive, but worth every penny. Takes damage and deals it like a pro. A single fighter can beat the game faster than some full 4 person parties, due to 4x exp
2) Red mage/wizard- this class gets very little credit. Also very expensive, more so even than fighter due to equipment and magic. It can use some good equipment, and has access to healing and black magic spells. So early on when black magic is useful for quickly killing groups, he can do that. Later when it's typically one or two big monsters a fight, he's useful using melee there.
3)Black Belt/Master- I'll probably get a lot of grief for this one but, when you have the fighter and red mage, the two most expensive classes, he's a great balance as the cheapest class in the game. Also, he's great for storing 4 extra weapons for super effectiveness against certain enemies.
4)Thief/Ninja- While he's basically outclassed by any of the above in terms of usefulness, he becomes quite nice when upgraded. A party of these 4, Knight w/Xcalibur, Red Wizard w/Masamune, Master w/Fists, Ninja w/Katana, was my personal fastest group for taking out Chaos. Also if you like running, he's useful for that, but I never run from fights. And finally, Ninja casting Fast(being that it's the best spell in the game) is quite nice.
5)White Mage/wizard- Magic is so limited in the NES version, that I hated the mages. Red mage was good because he could fight too. White mage was nice because he could heal, and later on I had him using the magic casting weapons like Thor's Hammer. That made him somewhat useful in battle, but a great help when I needed healing. Sadly though, he's completely useless compared to a Red Mage.
6)Black Mage/Wizard- I'll probably get even more grief for this one, but he's just so useless. The horrid defense means 1-2 hits from any good enemy and he's dead. The limited magic means he casts 2-3 spells and then, useless. Later in the game he can also use the magic casting items, but that makes every earlier spell he learned, once again, useless, except Fast. At least the White Mage could heal too. But come Chaos, all he did was cast fast on the fighter, use Nuke once or twice, then sit there doing nothing. At least I could have Red Mage or Ninja cast Fast, and then keep attacking as well.

so there
Fighter
Red Mage
Black Belt
Thief
White Mage
Black Mage

now in the GBA game, that order doesn't work, at all.

THE JACKEL