Muhammed Ali vs Georges St-Pierre?

GSP without a doubt. Ali only specialised in boxing - yes, he has hard hits, but GSP would be way to slick for him and once he gets Ali to the ground that would be it. I believe he would be able to get him to the ground extremely quickly, as Ali would have no experience in take-down defence, then he will be submitted quickly as he would have no ground game. It's a no-brainer in my book.