Fighter/Knight - Outside of expensive gear, there is nothing to not love about this job class. Best defense, second best physical damage and cool looking to boot.

Monk/Master - Best Physical fighter in the game with great HP to soak damage. These guys make boss battles a breeze once they get going.

White Mage/Wizard - All of the healing magic, enough said.

Red Mage/Wizard - Access to most of the Black Mage spells and some access to a few White Mage ones packaged in a decent fighter makes this a good versatile class. Not as good in games that fix the INT Bug though.

Black Mage/Wizard - I learned in my last playthrough that I have not been using this class properly. This is not a boss killer, most boss monsters actually have high resistance to offensive magic. No this class is all about getting through dungeons by blasting through the monster hordes in this high encounter rate nightmare. They basically make sure you don't spend too much time taking hits from random monsters and then use their useful buff spells in boss battles. This class is even better in games that fix the INT Bug and some of his spells.

Thief/Ninja - Thief is an absolutely trashy class but Ninja is actually pretty good though it loses some of its usefulness in the non-NES entries. The problem with Ninja is that it's a third rate fighter and mage and having access to a few of the useful Black Mage support spells is kind of redundant when a Black Mage and Red Mage already have them and more.