Frazier. He had one of the very best left hooks of all time, period, any weight class. Very few threw it as naturally and effectively.
currently the best short left hook in the heavyweights has to be Wlad Klitschko's he delivers it fast and from close range... ask Charles Shufford that same question ..LOL
Dempsey ruined his opponents whole face when he landed that hook, and frazier one was devastating wen he thre Ali of his feet. Tough call buh i'd go for Dempsey
I'm told Tommy Morrison, for all his faults, had a terrific hook. But Frazier is the obvious choice for being well-known.
Don't know too much about vintage fighters but I remember the left hook that Kirk Johnson KO'ed Maskaev with was huge.