Bob Foster It's really too hard to narrow it down to one, or just a few. Foster, Joe Frazier, Ray Robinson, Jack Dempsey, Ray Leonard, Felix Trinidad, Ruben Olivares...all had killer left hooks. Julio Cesar Chavez's left hook to the body was killer.
OMG how can't you add Roy Jones in there? WTF! He had a left hook that only Ray Robinson could match.
Tyson, Frazier, Cooney, Morrison, Chavez, Pipino Cuevas, Tua---no particular order but those guys could stop a fight with one hook.
Roy had a weird Left Hook almost a left hook/uppercut. It was great when he used to lead/leap with it.
Good mention, I forgot about him. Ray Mercer - he of the iron chin - said that Morrison hit him "like he had never been hit before." And Tommy's biggest punch was, of course, the left hook. In terms of raw power it is probably right up there with Tua, IMO.
In my lifetime, I've got to go with Morrison. That left hook is really what made him so dangerous fight in and fight out. All the other names mentioned are worthy choices as well.
Frazier, Dempsey, Liston, Foster, Trinidad, Cuevas, Morrison, Tua, Tyson, Jones, Robinson, Gatti, Carbajal, Patterson, Leonard, Chavez