No matter where you are in the game, there is pretty much always a Rage Gau can learn that will make him substantially more powerful than most, if not all, other characters in the game. Stray Cat is a good example - Catscratch is one of the most powerful any attacks any character can have when you obtain him. I don't remember what all the really good Rages were because it has honestly been close to a decade since I really played around with Rages, but there were quite a few of them. I do remember that Retainer's Shock is another fantastic ability. If he's at a decent level, it will wipe out pretty much all enemies at once in most enemy formations you can encounter for most of the game. (Unfortunately it's not available until near the end of the game).

I can't possibly rank these characters in order. There are too many great ones in this game. I will say that Terra would probably top my list though.