IMO its pretty clear that somebody like Pacquiao who is very fast, but southpaw has the best chance at beating Mayweather. Mayweather's one defensive flaw has been against the left cross, and against southpaws in general because he comes forward against them.

The reason Floyd is terrible for Mosley is because IMO he is still too good defensively for Mosley to really land anything effective, Mosley doesn't have a great jab like Oscar did to really stop Mayweather from throwing punches nor does he have Oscar's size or height. Also the two punches that land very easily on Mosley are the jab and the right cross (like Vargas, Forrest, and Cotto all landed with ease on Mosley).