I wouldn't mind him at United - he was poor last season but Martinez is simply awful at managing a defence. He knows he wants players to play, but that's as far as his instructions seem to go as far as I can tell. Under a manager who knows how to drill a defence Stones could flourish.
He is overrated, and he's not as good on the ball as some would like to suggest (just because he does it often, doesn't make him good at it) but he has plenty of potential to develop into something special. He just needs to get the right manager to develop him.