Ridley.
I know Sakurai made some BS size excuse.
I know he is a stage hazard.
But Metroid is a brilliant series (Other M never happened) with a rich history and some stellar games. Yet we only have Samus to represent it, and ZSS really doesn't hit me with that Metroid feel. You know who would?
Fucking Ridley.
Plus he's a flying space pirate dinosaur. Who wouldn't want to play that?