I just beat Final Fantasy V.

I hereby declare Mimes/Mimics to be the best spell-casting class in Final Fantasy. After mastering all classes except Mimic, my Mimics were built like this:

R.Hand: Lillith Rod (Magic boost, may cast Drain)
L.Hand: Wonder Rod (Magic boost, casts various magics)

Head: Gold Pin (halves MP costs) OR Circlet (Magic boost)
Body: Black Suit (Magic Boost)
Acc. : Protect Ring (Vitality boost, grants Regen status)

Skills:
Mimic (mandatory)
Dualcast
Summon
Time, Black, White, OR MP+30%

Dualcasting Bahamut, Flare, Holy, and Meteor. Mostly Bahamut. I owned the final boss before it could remotely come close to using its signature attack.