I'd say Holyfield had a good one. Not as powerful as Tua/Morrison/Wlad/Ruddock, and not as fast/effective as Frazier but it was a good punch. I remember him badly stagerring Lewis with a left hook. He lacked the energy to properly follow up at 37 years of age.
I always thought Leonard's left hook was on par with DLH's in terms of power, even though DLH's left hook was far more touted.
Ken Norton had real good hook. He might have broken Ali's jaw with it in the 11th round of their first fight. Kenny felt that was the punch that did it.
muhammad ali, not usually regarded as a puncher and most discuss his jab and cross but i feel like he had a hurtful and accurate left hook when he say on it the fight that always jumps out is frazier 3 for his left hook
Among HWs I immediatly thought about Jeffires - he was rarely mentioned among the best left hookers of the division but he made career out of this punch.