The last few times I've played through, I've given special treatment to Terra, Celes, Locke, and Shadow. With Terra and Celes, I max out their magic power by leveling them up with the right espers, and teach them every spell. They can also use the best equipment, too. So they are my most powerful characters.

I like to level up Locke and Shadow (and sometimes Sabin) with Odin until they have very high speed, and then boost attack power after that.

In Kefka's tower, I can put a super magic user in each group (Terra, Celes, and the other is usually Strago), and a very fast fighter. At a high level, that's really all I need.

One time through, I made Mog my most powerful character... he had max magic power and every spell, and he kicked ass. That was fun.