I'd favor Chavez, Armstrong, Duran and Pryor based on match-up of styles. Loi, although not much of a power-puncher, was a VERY slick high-volume puncher, so he might well be a bad style match-up, too. Locce and Whitaker are 50/50 fights. So is the DLH that fought JCC and Gonzalez at 140lbs. I'd say Napoles, too- but his tendency to cut prevents me from picking him outright. Ross and Canzoneri might be considered threats, too. And, although I don't favor him, KT would always be in the fight due to his power. There were a lot of really, really good fighters who graced Jr. Welterweight, and I don't think PBF would be a prohibitive favorite over any of the fighters mentioned.
What? I love Pernell. The man was a ****ing boxing genius. I have him in my top ten boxers ever. I just wish the fight would have taken place in 90 like it was originally planned but Pernell's people didn't take the fight. Not saying they were scared or anything. Just would have been a way better fight.
Duran would likely edge out Mayweather at 140lbs. Even though Duran never stopped at that weight to pick up a title, we can safely say he would have been effective there. Duran did fight a few non-title fights at 140lbs, but its not a division his career had any distinction with. Duran would not have an easy time with Mayweather. Some classic forum guys might think so, but I'm not entirely sure it would be an easy nights work for Duran.
Whiatker would have beaten Mayweather at 140lbs. And his piston right jab would be his main weapon when it came to getting the better of Mayweather. It wouldn't surprise me to see Whitaker being the aggressor.