Joe Frazier- A lot of people believe he actually won 2 out of the three fights with Ali. I think he got robbed in the second fight. But you knew going in that, even if you had an edge, Ali would get the decision. There were quite a few fights Ali could have easily lost, but the judges scored in his favor. Which is why I believe Ali was the most over rated athlete ever and Joe Frazier was one of the most underrated ever
The admirable thing about Ali is that he ducked nobody. It led to his downfall in later ages. His boxing skills were not overated (perhaps your referring to his power). He was smart, quick and had stamina like no other. He never ever quit in a fight, even when he lost. I think his biggest attribute is that he showed no fear (even though I'm sure he had it). Ali was a promoter, bragger who simply could back up his words. A lot of people really hated him, but he just wouldn't lose. He was a showman in and out of the ring. He may not have actually been the greatest, but he sold the World that he was, and the world (including myself) believed it.
As for Frazier, well he was just an animal. A pit bull who seemed to think every round was the 1st round, in other words he was relentless. He farked Ali up bad and accelerated his demise. Prime Frazier vs Prime Tyson would have been probably the greatest fight ever.