Best: Warrior, Thief, White Mage, Black Mage

With this team you can equip every spell (though not all at once because of the three spell slot limit for each level) and every weapon and armor in the game (excluding bonus dungeon equipment). It's also really good when it comes to variety (two fighters and two mages, two white magic and two black magic users).

Worst (taking into account the fact that I don't consider solo, duo, triplet teams or repeats of characters in the same team): Thief, Red Mage, Monk, Black Mage

I've used this team twice and I guess it's still rather balanced. The reason I consider it the worst is because the Warrior and White Mage are extremely vital early on in the game (Warrior for being at the front taking all the hits, white mage for being able to access vital curing magic and status ailment healing spells). Also, there's a lot of really good equipment that remains specific to the Knight, even after getting a Ninja (especially stuff like Excalibur and the Dragon Mail). As for the white wizard, the red wizard won't be able to use spells that are really vital in a pinch (such as Full Life and Healaga).

I was wanting to make a thread exactly like this, except with my "full team of four and no repeats of classes" rule, but I guess I'll save that one for later since this thread is active.