Shane Mosley's career = Somewhat of a Disappointment

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The guy's never been stopped (proving one of the best chin's in boxing history), stamped his legacy as arguably the greatest lightweight in history along side Roberto Duran, gave Oscar his first true loss when he was in his prime, and went after every fighter the boxing public wanted to see him in the ring with. The words 'Mosley' and 'Disappointment' do not belong in the same sentence.

Who did Mosley beat at lightweight to deserve recognition alongside the original Leonard, Duran, etc.?

His list of opponents does not equal Duran's (don't know why you said Leonard as he fought virtually his entire career at welterweight and above), but Mosely essentially never lost a single round as a lightweight and is universally considered as the only 135 lber who could have possibly beaten Duran.