I know I'm gonna sound like just another stupid fangirl for saying this, but, Axel and Saix are my favorites for Mission Mode. In my opinion, they have the perfect balance between speed and power. For instance, Larxene is uber fast, but, for me, it gets tedious to try and kill enemies and Lexaeus (sp?) is really strong, but i cant stand how slow he is.
But what I've found (and as it seems, this thread is only proof of that) different people are going to say that different characters are good or bad based on their personal battle styles. Just like you said u didn't mind Demyx, Sacred X, but, personally, even though I like him as a character, he sucks as a fighter. And thats mostly because i prefer not to fight with magic. Like I said, that's just my battle style.