When talking about heavyweights, I'd say Sonny Liston Joe Frazier Mike Tyson Jack Dempsey Tommy Morrison Cleveland Williams Joe Louis They are all in no particular order and they are from the top of my head so it is possible that I've missed someone that deserves to be mentioned.
Sugar Ray Robinson Joe Louis Joe Frazier Bob Foster Ruben Olivares Mike Tyson Donald Curry Felix Trinidad Nonito Donaire
Bob Foster ( Don't believe anyone had as many "unequivocally over" ko's than he did off the left hook.) Joe Frazier (a left hooking machine, could counter with it, lead, double and triple hooks wasn't the most powerful, but the greatest left hooker at heavy overall.) Joe Louis (speed precision, and power) Chavez (his body punching with the hook was devastating) Olivares ( The Mexican hook to the liver was his specialty, he may have created that legend.)
Michael Spinks. He's known for his right hand, but his left was no joke either. It was the left hook that sent Marvin Johnson to dream land.