Duran was the superior fighter overall. Ali defeated the best competition in heavyweight history. Were the fighters Ali defeated better skilled than Dejesus, Leonard, Benitez ,Hearns or Hagler? Doubtful, and no.
Both have fantastic resumes, but Ali never bitched out in a fight. That's the difference make for me. No Mas is just so unbelievably bad.
I always look at Muhammad Ali's career like this: what if he hadn't had a break in his 3.5 - year career. He would certainly be even more successful. So I vote for Muhammad Ali.
Duran. Much longer and successful career, starting from an ATG in his natural weight class and winning belts as he advanced up including achieving the rare old beats young for a title. Can't really hold the Leonard and Hearns losses against him as he redeemed himself every time people casted doubt. Ali just overshadows him due to HW popularity over other weight classes and his persona outside the ring. Also had some questionable or gifted wins throughout his career. Another thing, Duran was voted fighter of the 70s, the same decade Ali regained his title against Foreman and fought practically everyone in the HW division.
But they would have all beaten DeJesus, Leonard Benitez, Hearns and Hagler. I don’t think that’s doubtful and no.