Manny.
Floyd has yet to fight an opponent with even close to a power-to-speed ratio that Pacquiao has. All his marquee matches were with guys incredibly past their prime, like De La Hoya. Mayweather also has zero punching/knockout power. He doesn't even try. He reminds me of a poor man's Sugar Ray Leonard, except that Leonard could punch a bit harder. Leonard would win against physically superior specimens like Hagler and Duran, even though he couldn't out-box them at gunpoint (Leonard's decision over Hagler was arguably the biggest boxing robbery ever). Mayweather is incredibly similar.
The big problem I have is that Mayweather is, without question, the best defensive fighter in the middleweight class today, and probably top 3 ever. He also puts on a show. Because of this, Pacquiao has to either knock him out, or look so dominating that the cards couldn't possibly split to the favor of Mayweather.
I have faith that he can do it.