I say Floyd. I think that's what he will catch Cotto off guard with. By the mid rounds, Cotto will be on the lookout for that laser right hand, Floyd will probe out his left like a jab and snap it around Cotto's right glove or thread the needle to his chin. The icing is that Floyd will throw this shot while stepping way off to his own left, both body and head TOTALLY out of the way. Think of that knockdown against JMM
Also, to be clear, doesn't have to be of "all time". We can go ahead and say "best in recent memory" or 20 years, you get it.
And also, in this poll atleast, I make no distinction between a "hook off a jab" and a "hard-feinted jab to a quick left hook".
Lamon Brewster deserves a mention.
I can't think of a current fighter who relies on it that much. Floyd does it pretty well. Andre Ward can do it too. Maybe Kendall Holt but I can't really remember with him.
Joe Frazier, David Tua, Roy Jones, Julian Jackson.
Active fighters i'd say Nonito Donaires, Wladimir Klitschkos.
Rakhim Chakhiev possesses an absolute beautiful left hook he throws a southpaw stance.