My first time through FFII

[Cyric]

Ultima draws its strength from all of the casters weapon and spell levels. It's actually quite powerful if you bother to level everything up.

Well, apparently in the NES/Famicom version, it's bugged. HCBailly mentions it between 7:00 and 8:00.
YouTube - Let's Play Final Fantasy II #35 - Ultima!