I think theres no real argument if you take his career overall. He got lucky in that there was so many good fighters about in his era. What enhances him further is he didnt actually have that much difficulty beating liston or foremann considering their form coming in. Regardless of what people say...liston was actually a great fighter. I still say without ali both foreman and liston we,re looking at longish reigns. He really did mess their careers up. More so liston bearing in mind foreman came back years later. But he derailed his first career anyway. Ali was great in the seventies. But the greatest was really back in the sixties. That really was poetry in motion. He cemented himself as the greatest in the 70,s but showed the best skill in the 60,s. Regardless tho...taking his whole career into account he deserves the accolade. Not only did he beat them...he often predicted the round he was going to beat them in. It was effortless stuff. Watch some of them fights again...just masterclasses from start to finish. For me that was the most gifted heavyweight the worlds seen before or since. Marciano frazier foremann liston louis tyson holmes lewis holyfield whoever else you have on your list...they we,re all great fighters. But not one of them comes close to having the natural ability that ali had for me. Just a rare talent the likes that dont come along very often.
Ali is the best heavyweight ever, but not the p4p most complete and accomplished boxer ever. That distinction probably goes to someone like Ray Robinson, Henry Armstrong, or Harry Greb.