Frazier...Cassius Clay didn't have the strength and savvy of the mature Ali to adequately deal with FOTC Frazier.
Probably Frazier. I think the Ali of late '66 and early '67 (just before he stopped fighting due to that army draft thing) would have beaten Frazier though.
As it stands, Muhammad Ali gave Joe a great fight the first time around then beat him in their next two meetings. It only stands to reason with me that without the layoff he only does better. I've never bought into the notion that the 70's saw a vastly improved and gracefully mature version of Ali. What I see on film is a guy who holds behind the neck, leans on the ropes and using various tactics to conserve energy as a result of lost ability. In the 60's, Ali was noticeably quicker and more importantly could keep up that speed and tempo for 15 rounds.
Frazier. The young clay was less secure on his feet, had less upper body strength. Frazier would have out-slugged him.
Mental talk. Cassuis would have won it. Was a far better fighter pre lay off. He slowed down a good bit after it.
Clay with consummate ease, Joe would be hitting nothing. To quote Clay from around that time. " Your hands can't hit what your eyes can't see "
Any version of Ali would have been given hell. The reason being, he would underestimate Frazier first time around. He wouldn't know what Frazier would bring until he was in the ring with him, then it would be too late. Although saying that, Ali wasn't the greatest at figuring a fighter out in rematches.
Frazier for the reasons mentioned above, plus he definitely had 15 hard rounds in his tank...we don't know if 1964 Clay had that... I've always felt it significant that of Clay/Ali's 4 main rivals (Liston, Frazier, Norton & Foreman) the 2 he had the most trouble with, Frazier & Norton could do 15 hard rounds, whereas the 2 he beat more convincingly, Liston & Foreman, appeared to have significant stamina issues. Not a coincidence, imo.