Sure, Ultima played the role of the "great magic", but its strength was disappointing: METEO1 actually dealt more damage than ULTM12.
That's actually due to the odd damage system for Ultima. It draws it's power from your other weapon and spell levels. I have it doing at least 4000 a hit, against enimies in pandaemonium.

It depends on the game, but in general, I'd say Ultima. Plus, it looks cool.