Just about every Square Enix RPG boasts at least one ultra-strong mage. From the white mage archetype known for healing and curing ailments to the red and black mages known for dishing out damage, there are plenty of examples of Square Enix spell casters who wield serious power.
While it would normally be quite difficult to compare mages across various titles, Final Fantasy and its spin-offs all tend to pull from the same pool of spells, which actually makes it a relatively simple task. If a character has access to some of the most memorable spells in Final Fantasy, there's a good chance they're quite powerful, and the opposite logic applies. Of course, there are other magical feats, in cutscenes or otherwise, that may rocket a mage to the top of the list.
Kefka Palazzo (Final Fantasy 6)
Kefka is without a doubt one of the most powerful villains in Square Enix history, even rivaling Sephiroth. A failed Magitek experiment turned court jester, Kefka is often regarded as the very definition of insanity, and he also happens to wield an inordinate amount of magical ability. At the conclusion of Final Fantasy 6, Kefka manages to transform himself into the literal God of Magic, a feat that places him comfortably at, or at least near, the top of a list of the strongest mages in Square Enix games.
While Kefka's interesting lore goes a long way in explaining his distinction as one of the most powerful magic users, Kefka's in-game feats are equally impressive. In the final fight, Kefka rotates between using the three most powerful versions of Fire, Lightning, and Blizzard spells, but outside the final encounter, Kefka also employs an extremely powerful spell known as "Forsaken," which is unblockable, deals a ton of damage, and applies Silence.
Donald Duck (Kingdom Hearts)
Yes, Donald Duck boasts unparalleled raw power in spellcasting. On the surface, Donald Duck is just that quirky Disney character that fans know and love, but his feats in the Kingdom Hearts series put him way above far more serious protagonists. Not only does Donald have access to the most upgraded versions of Fire, Lightning, Cure, and the like, but he casts a specific spell in Kingdom Hearts 3 that sets him apart from the crowd.
Donald is in a very small group of characters that have cast Zettaflare. While one of the other casters could technically qualify as a mage, their repertoire is limited by comparison to Donald's and the other is Bahamut, a gigantic dragon god. All in all, that's not bad company to keep in terms of spellcasting ability.
Ultimecia (Final Fantasy 8)
The Final Fantasy franchise introduced three of the most powerful magic users in the series' history all in a row: Kefka, Sephiroth, and then Ultimecia. The story behind Ultimecia is that she wants to use her magical ability to compress all time into a single moment and rule over it—a totally evil idea but an impressive one by spellcasting standards. It's difficult to compare Ultimecia's power to Donald or Kefka, because she focuses almost exclusively on Time magic throughout Final Fantasy 8, but her feats, like almost achieving her goal of compressing time, are more than enough to distinguish her as a Square Enix powerhouse.
In terms of her in-game ability though, Ultimecia has access to the spells that almost all the most powerful Final Fantasy villains use. She can cast Ultima, the most powerful spell in any given Final Fantasy game, as well as Meteor, Flare, and Apocalypse, which are all extremely hard-hitting spells. Technically, since Ultimecia imbues her powers into Edea towards the end of Final Fantasy 8, they should share this spot, but regardless, Ultimecia is one of the top mages in Square Enix history.