He was the greatest lightweight of all time, beating every man he fought at lightweight. Viret, Bizzaro, Ishimatzu, Buchanan, DeJesus, Robinson Garcia, and Ernesto Marcel all great wins at lightweight alone. He could have retired before he even fought Leonard and been recognised as an atg fighter, remarkable to thing about how much he did after moving up.
The losses above LW are virtually meaningless. The wins are astounding. He was a LW, ffs. Duran was a bad, bad man.
He was the best lightweight ever, so the fab four fights beyond his best weight were just the topping on his already ATG career and they were basically all win-win situations where his defeats would not be held against him like they would for a normal fighter. I believe he won a MW belt 19 years after winning a LW belt, and came back more than once with significant victories after being written off, so it's his achievements to continuously astound which also props him up. His defeats actually help him there.
In the latest issue of ringmag, Duran was listed as the best fighter of the 70's. Ali was listed at # 8.
Doesn't take much luster off the accomplishment though, I'd say. Look, Duran is the greatest fighter of the last 50 years. Anybody who doesn't believe so can suck a fat one. No, in all seriousness he has just about everything required: The skills, the resume, the H2H ability, divisional dominance, P4P accomplishment, longevity... all in spades. Easily one of the most ferocious and fearless competitors to lace up gloves.