Ali has that title and what gives him that is he was the greatest, then was out of boxing for like 3 years because he refused to go to the military, then came back to win the belt again. I think George Foreman is in the Top 5 simply because he was good in his prime and still came back at his age to win a belt against younger, quicker boxers.
Tyson should have been the greatest of all-time, but Don King messed that up. If Tyson kept all of his original trainers who saw him for more than just a paycheck, he wouldn't have been in as much trouble as he's gotten into and would have stayed more focused on boxing. In Tyson's earlier days, he use to studie old boxing film of old boxing greats as if he had Finals coming up. After he married Robin Givens and she pulled in Don King and his people, that was the beginning of the end.