I think most people think of three people when this subject comes up. Pep, Whitaker, and Floyd. I think of quite a few more including Benitez and Duran among others, but I will just address the three most popular choices.
of the three, I think that the order is Whitaker, Mayweather, and then Pep.
and here is why.
Pep is third in my book because he did not move up and test himself against the physical disadvantages that both Floyd and Whitaker have, though he did test himself against Sandy Sadler who should have been at least a welterweight.
I have Whitaker over Mayweather because Whitaker demonstrated his defense in higher output situations. Part of Mayweathers game plan is to slow the action down to 30-40 punches per round. Whitaker demonstrated his defensive abilities vs. opponents that were chucking 70-80 punches per round.
When floyd is faced with that kind of pace, his form breaks down and the fighter in him comes out as we saw with emanual augustus, J. Chavez, and JC Castillo.