i can't believe nobody mentioned FREEZE... it's not super strong, but it freezes the enemy almost every time (at least when i use it) and u can use it, attack with anything else and if the monster becomes moving again you can just cast it again lol i love it.

Also i enjoy MIME with KoTR of course. too easy and too long but hella strong.
and i never really used Ultima that much. i didn't think it was THAT strong.... but maybe that was just me.

I thought Shadow Flare looked nice and has an awesome name but wasn't that necessary in any battles or anything like that.

i'm sticking with FREEZE