I want a bad guy who's not like the other bad guys.
He/She should be ALWAYS relentless.
Kefka was sort of like that.
He/she should also get what they want to finish their goals then bring the world into chaos.
Makes the final battle seem better in my opinion.
Oh, and they should never have another Kuja and Seymour.
Also, it really depends on what the story is about in the case of monsters or human villians.