IMO, I think that Ali would have won. As Jack eloquently stated, he was the quickest and could take a punch. Btw, he was also very smart, could adapt, & punched hard as well.
After his trainer/mentor Cus D'Amato died, the myth of Mike Tyson was exposed when the giant potato head (Buster Douglas) knocked him out with an aggressive arsenal of punches. Up until that time, everyone had either fought cautiously (ran) or stood up straight against Tyson. Subsequently, Evander Holyfield who is really a pumped-up powerful Cruiserweight with a good defense, drove the point home when he came after Douglas with a relentless barrage of punches from all angles and beat him savagely twice. He must have been enraged by all of the pending paternity suits- Tyson just happened to be there.
In fact, I think that there were a slew of other AGGRESSIVE heavyweights that would have beaten him or at least given him a run for his money- Joe Louis, Marciano, Jersey Joe Walcott, Joe Frazier, Joe Foreman I, Riddick Bowe, and don't forget how Lenox Lewis embarrassed him.
Ooooops, almost forgot MACS the punching machine. (c;
Btw, the caveat to the aforementioned is that boxing is a corrupt sport reliant on human judges and a referee who can be swayed...