Which 4* should I upgrade to R4?

I recently double duped Gambit with the PHC and 4* so I have a lot of mutant ISO to use up, but I'm guessing he's not the best champ in this group?

Comments

Gambit has pretty much zero utility. He's fun to play, but has nothing else going for him, that I can gather.

That's what I figured but I wanted to make sure I wasn't missing anything.