Tough question because my answer would be Joe Frazier who was an undisputed and undefeated champion who made numerous defences of his title. Joe gave Ali his hardest fights and made him his nemesis (which was not easy considering all the achievements Ali had).

Now you could say but Big George Foreman beat Frazier easy twice but styles make fights and Joe was past his prime.

George Foreman would be second, biggest puncher and such a powerful man who became champion at over 40 years of age.

Liston would be third who was also intimidating and powerful champion.

Lyle, Shavers, Norton, and Patterson would be on the list.

Holmes beat Ali so does not count. Larry is my 3rd best heavyweight of all time.