I agree on everyone except for Duran. I actually rank Gans and Leonard ahead of Duran at 135. I could see ranking Duran ahead of Leonard, but Joe Gans is simply the best lightweight I've ever seen, and his resume, while a bit weaker than Leonard's, is on par with Duran's. Head to head I don't think anyone in history could beat a prime Joe Gans, and that's just based on the relatively limited footage I have of him.
I agree with Pep as the GOAT featherweight, but Armstrong could make a claim too.
Sam Langford is extremely hard to rate at any one weight class, but he may have been at his best at welterweight... and maybe as good as Robinson. Not enough footage of either man at welterweight to say.