My money like most would be on Charles but a prime Leon was a lot better than most people on here think he was. A focused well prepared Leon was a handful.
He was more than that. He won Olympic gold in 76' against all of those strong Soviet Union/Cuban fighters. That was a time when all of the best eastern Europeans stayed and fought as amauter due to communism. Gold in the Olympics and Silver in the Pan American games is nothing to sneeze at especially in those days. He could ha e had a much better pro career had he been able to control himself outside the ring. Great pressure fighter that was tough as nails. He could have been so much more as a pro had he kept the same focus that he had as an amateur.
Leon was a good fighter, very good on a few occasions. Even at his best however he was clearly nowhere near a great fighter, let alone all time great. He had tremendous courage and until his lifestyle sapped his abilities he had a great engine; very active, always punching. Assume we are talking best version of each Charles wins this any way he wants. If he wants a KO he will get it, else he should be able to pitch a shutout. Leon didn’t have the power or the size even to trouble a smaller heavy like Ezzard. Even at his best Leon struggled mightily with the likes of LeDoux, Lopez, and Evangelista. I really like Leon. He was a character, but this is a mismatch
Leon was a prospect when he met Ali but he was rushed and stopped developing after that. Too much too soon. He could have maybe been a good cruiser he was too small for the bigger fighters that were emerging. Prime Charles outboxes him. Gets unanimous decision.