Benny Leonard was the greatest lightweight of all time - by a long way
Esteban De Jesus (x2)
Edwin Viruet (x2)
Jack Britton (x2)
Lew Tendler (x2)
Freddie Welsh (x2)
Soldier Bartfield (x3)
Johnny Dundee (x4)
Rocky Kansas (x4)
Doesn't this just speak for itself?
Yes, Leonard lost every once in a while, but you're talking about an era where you might fight four times in as many weeks. Nobody had a record like Leonard's back then apart from Harry Greb.
You can say Duran was better, and use that as part of your criteria - but if accomplishments are what you go by, there's no denying Benny Leonard.