Kariya probably had better potential as a magus than Tokiomi, by far. Considering Uncle Kariya would've been perfectly fine with a non serker Lancelot, the man had some serious potential. He just didn't have the proper experience or abilities as a magus, and Zouken also trolled him for Zouken reasons.

In terms of potential, he was probably the second strongest magus in the war, behind Kayneth. Kayneth was easily better than the others in potential and magus capability and such. The worst was probably Waver, because he does sort of suck, though I guess technically he's equal to Kiritsugu on the magus scale. That isn't saying much.

Eitherway Tokiomi was pretty bad, he just had a good work ethic and trained and trained to make up for being bad. Ironically enough in a way he embodies Wavers ideas more than Waver despite being a conservative Magus.