Best (as in most effective punch): Robinson Most powerful: Louis, Frazier, and Jackson gotta be up there.
https://www.youtube.com/watch?v=WF8dcFvMEJs by the way, Klitschko has won several fights via left hook KO.
Having the best hook in history should take into account different factors. Not merely power. This is why I would pick SRR.
My favorite left hook is Chavez sr. It may not be the very best, but its surely up there amongst them.
Captain Hook! end of story! Never seen anyone that can fire of 3 and 4 so fast, or hit with a deadly one, when at what should be a safe distance.