Sugar Ray is well remembered for his dominance at welterweight ahd middleweight but he began his professional career as a lightweight. So if he fought Duran at this weight will it be like Hearns vs Duran or like Leonard vs Duran?
Robinson's speed is too much for Duran and at the weight he really should have been a world champ when he beat Angott at 136lbs. This was agreed so Angott wouldn't risk his title. If Robinson was actually trying to make the lightweight limit he could have easily have done it up until the fights where he was 145lbs. But in those days they didn't weight drain like later generations as they often fought every 2 weeks.
I don’t know how long Robinson fought at lightweight. As someone already mentioned he might have been too green or simply not developed enough at that weight. Roberto Duran has a very strong argument for being the best ever at that weight or easily one of the top three. My inclination would be to favor Duran.