Were talking about how good someone is at the weight they fight at. Duran/Napoles are close in class but the difference is Duran is proven above WW, Napoles is not in any way. If Napoles is a great win for Monzon, doesnt that make Marquez a great win for Mayweather? Both are undersized greats and only have 1 fight at the weight against a significantly bigger opponent.
In fairness to Sanchez, his career was taken away from the sport by tragedy. Both men fought quality opponents, but I'd give the slight edge to Monzon. He probably could have fought a young Hagler in the twilight of his career, but the wins against Valdez, Briscoe, Benvenuti and Griffith set him apart. The wins against Griffith are slightly questionable because Emile arguably lost his killer instinct after Benny Paret's death. Who's to say what would have happened had Griffith fought Monzon with the same fire that he had earlier in his career, but I say it's a safe bet that Emile would have been a stronger fighter mentally. He just didn't have it against Monzon.