Not my favorite heavyweight, but an obvious choice for the greatest of all time. I didn't like the fact that so many of his punches were thrown while moving backward, and he seemed to lack real power. Not that there wasn't some sting on his punches--I'm sure there was--but just lacking the authoritative power of a Joe Louis or Jack Dempsey.
What he did have was real speed--an athletic phenomenon. He used to brag that he could throw a punch and hit you before you could see it coming, and I believe him. This brings up the question "if force=mass X acceleration, why didn't he have power?" Maybe because of a more gracile build, one that allowed that speed. He was also remarkably elusive. He seldom took a clean shot, and when he did he showed a good chin.
If he'd had that power, then I'd say he was the greatest. Its not clear to me. My own candidate for GOAT, Louis, suffered from inelegant footwork sometimes.
Its common when comparing fighters to look their record, but this is a dead end. The measure of a fighter is who did he lose to. Here Ali can't be measured by his end of career losses to Trevor Berbick and Larry Holmes, but by his loss to Joe Frazier. Both were at or near their natural physical peak. Frazier would have been champion no matter what era he had been born in. It took someone that good to beat Ali. However, the annoying fact that Marciano never lost just won't go away, and tends to render record comparing useless. One can only try to objectively evaluate their skills.
I think its a mistake for boxing commentators to interview boxers. They are often painfully inarticulate, and the contrast between their verbal fumbling and graceful display in the ring can be stark. But Ali had something to say. Begining early in his career with entertaining remarks, he evolved his interview into a message. Essentially, what he said was he would win because he had a better character than his opponent. His victory was a consequence of being a better person. And we believed him. By his efforts he raised boxing from a sport to a form of theater.
Thinking is overrated