Frazier but by decision. Probably the best possible heavyweight matchup in history in terms of sheer excitement.
I agree with Sly that this would easily be one of the best HW fights in history. Both guys could bang. Both guys had tremendous chins and resolve. Both guys only knew one direction, forward. However, I think Joe's left hook would be the most dangerous weapon in the ring that night. I don't think Marciano ever faced that type of punch, and I don't think he'd stand up to it over 15 rounds. Frazier wins a bloody contest via TKO in the 14th.