Jeff Clark's 1914 victory over Sam Langford should be in the running, but there's no film and not a lot of reportage. But Clark was still a LHW which makes him the only non-HW to beat Langford over 10 rounds or more in his prime. No judges though, and it's points, so it's iffy. Tommy Gibbons is the only man to have ever gotten the best of Harry Greb in his prime, and that really is a sensational victory, it's well documented too. Gibbons did the business in 1920. He gave Greb a good, good hiding, put the kind of beating only a heavyweight Gene Tunney was able to replicate against the one-eyed, past-prime version. Again though, newspaper decision, no footage. Fritzie Zivic's beatings of Henry Armstrong in 1940 were impressive, again, you can't see these. Duran's win over Leonard is probably as good an answer as any that were filmed.
There's this too: This content is protected Robinson wasn't prime here, he wasn't taking Turpin all that seriously, but it's the closest to his prime at which he was beaten. Special stuff.
My vote too. Too significant worldwide at the peak of boxing popularity. Maybe not as remembered as Zaire now, and not as big an upset, but that's the one to me.
This is more in line with my thinking when I see "What was the greatest win in boxing history?" To me, we should be considering the greatest fighters who beat the greatest fighters. I had Harry Greb in mind, too, but not for his loss to Gibbons, though that is a great call. I think his victory over Mickey Walker is monumental. A post-prime, one-eyed champion, beating one of the p4p greats. I would add: Leonard vs. Hearns I, Pep vs. Saddler III, and Britton vs. Lewis (1919, where Britton knocked Lewis out for the title).
There are many good candidates listed, but I still stand by the idea Ali's win over Foreman is the single greatest signature win. A prime undefeated power puncher who just crushed two of the men who beat Ali, the press including his long time friend Cossel expressing doubts and being filled with fear, his own entourage in the dressing room allegedly looking like they were about to attend a funeral...it doesn't get much scarier and nerve wrecking than that. I can't imagine the sort of anxiety that went through Ali's mind. And then to actually take the kind of punches he did in a very risky gamble on the ropes, sacrificing his body to protect his head, the physical agony is hard to imagine given the footage of Foreman leaving dents in bags like he had cinder blocks in his gloves. And Ali was past his prime on top of all that with diminished speed. And then the boxer win by KO over the slugger...just poetry. You couldn't write a better ending. The fact Foreman would make a come back to recapture the title in a second sensational career just adds to the merit of the win.