Here are things I think he could exploit

1. Thomas Hearns: Mayweather could get in on TOmmy, and Tommy can't fight on the inside. I think that at Welterweight though he hit hard Tommy wasn't physically strong in that he couldn't push around boxers. I say also that Mayweather could eventually knock out Tommy with an accumalation of blows... especially at Welterweight where Hearns had no legs whatsover.
2. Duran: Too hesitant, and defensive, Mayweather would be able to pick him off on the outside. Eventually Duran would get frustrated and either quit or become wild and Mayweather would pick him apart even more.
3. Leonard has amazing reflexes, and speed, but he doesn't have great defense... He would be like watching the old Roy JOnes Jr. who didn't have defensive skills as much as athletic ability. THe problem is that Mayweather has both, and he slips seemlessly from offensive to defense and vice virsa... Leonard would have a very hard time hitting Mayweather, and footwork would eliminate his combinations which I think Mayweather could avoid for the most part on the inside anyways. And though Mayweather is as hard of a puncher as Leonard he is more or less as strong.
4. Hagler: would walk through anything Mayweather could throw, and would probably knock him out with body shots.
5. Pryor: isn't as good as Duran IMO, and would have perhaps even more problems than Duran with Mayweather, though I don't think he would give up as soon against Mayweather, and it would be a tough fight which I think Mayweather would win the majority of the rounds.

Here are reaons I think he would lose.
Hearns: great boxer on the outside, good speed/reflexes, and hits too hard for PBF to handle from what I've seen with his right cross landing cleanly.
2: Mayweather doesn't use his legs as much as Leonard at welterweight, and Duran is quick in his own right so Mayweather may not be able to get away from Duran like did, also he doesn't hit as hard as Leonard so he may not be able to keep Duran off him.
3. Leonard is almost as fast, but a natural welterweight, and is a harder hitter than Mayweather... If Mayweather tried to prove himself by comming forward like he did against Corley I think he would lose.
4. Hagler: wins pretty much everytime how I see it... he is too big.
5. Pryor is relentless. Can take anything PBF can dish out, and he cracks like mofo.