I think Paul Williams beats them all because of his height, reach, workrate and his ability to take a good shot. He will overwhelm any welterweight. He probably can't make welterweight anyways.
Cotto will be able to break Mayweather's defense with his strength and skills.
Mosley is big, fast, and fearless. His downfall is his age.
Manny PAC will get destroyed by Mayweather. I think Cotto is going to knock out Manny. Cotto is used to fighting the big boys, Clottey, Margerito, and Mosley.