Duran by a mile. Then Monzon, just ahead of Arguello and Jofre. In a p4p sense, probably Langford though, but I haven't seen enough of him on film to put him above Duran.
Prime Mike Mccallum was an underrated great. Duran and Ricardo Lopez should be mentioned as should Lennox Lewis also. I would probably say duran as far as legacy goes. He lost to the 3 best fighters he ever fought, but he did beat SRL once, gave Hagler hell, and upset Iran Barkley when he was pushing 40. Not to mention he's one of the best, if not the best lightweight ever.