His best moment was a single punch out of no-where while getting hammered for a long period both before and after. I'm thinking not only 15, but 15 against a great fighter. There would be a big difference in going 12 vs the Smith's and Tucker's and 15 against the Ali's.
But Ali couldn't deliver the sort of punches he took. That is the important part. And once Wlad tamed his anxiety he was able to take some monster punches, as he did most recently against Joshua. Williams and Foreman were decent sized heavies but no monsters.... and Williams was on the road to shot while Foreman was mentally fragile (at that time).
The word prime means what? Under 32? If that' the case start with Ken Norton or Joe Frazier. Past that timeline, Holmes, Lewis, and both Klitschko's. Ali had problems with good jabbers at ALL stages of his career, and preferred guys to come to him. If he's facing a taller man with a greater reach, Ali is going to have to come to them, something he really wasn't comfortable with but did at times when smaller guys were scoring on him