I think the best party consists of

Fighter
Fighter/Black Belt/Thief
White Mage
Red Mage

This depends on your playing style, though. Offensive magic is so limited, at least on NES, WSC and PSX versions, that I prefer having a Red Mage instead of a Black Mage. You'll still get the basic Fire/Lit/Ice 2 for mobs, Fast/Haste for buffing physical attackers, some extra healing and an additional physical fighter.

In a Fi/?/Wm/Rm party, the [?] part is a tough choice. Another Fighter would provide more attack power and armor, but having two characters of the same class is arguably rather uninteresting. Black Belt is a very good choice. You'll get great attack power, and his low defence shouldn't matter since you can make Red Mage your 2nd character. When you keep Black Belt as your 3rd character, he should stay alive pretty well. Thief is another good choice if you want to sacrifice some offensive power for better survivability. Again, Thief is quite weak during the early game, but that's why you keep Red Mage as the 2nd character in your formation. A nice part of this team is that the characters make up for each other's weaknesses: Red Mage/Wizard is good during the early game but gets less powerful during the late game, whereas it's the other way around for Thief/Ninja. Finally, with two healers (three after class change), all those defensive spells and Thief being good in escaping from battles, this is a party that is very hard to kill.