Let's see, the only particularly useful early magics are Big Guard and White Wind, both of which are equally useful no matter who applies them. Other magic quickly loses its usefulness for a good period once you step outside Midgard. Especially since it has roughly equivalent damage with just plain attacking, but also costs MP.

And Cloud is actually the best mage, once equipment is included in the consideration.
Though speaking of mages, Cait is the second best natural mage. Does anyone ever replace Aerith with him for mage purposes?