that´s very easy. frazier beats him again (probably on points). most of us consider frazier in the fotc as the best frazier ever; talking about prime frazier. the majority of this forum´s members will also agree that ali was much closer to his prime (which we never saw) in 1971. in his comeback fight in zaire ali was considered "over the hill", and, after that, slipped more and more every year. vice versa: following my logic, manila frazier would lose against close-to-prime new york ali.
1971 wins twice. Both were better then. The true hypothetical 'classic' between the two would have been 1971 Joe v 1974 Muhammad. Reason being that,obviously, the former was at his best over the three bouts in their first battle while the latter was in the second one. In Fight II Ali still had the speed that he had in FOTC but was sharper and in better shape.
Madison Square 1971 was Frazier's most magnificent performance.There's no way he could go but down after that one.