I see Julian Jackson being mentioned and whilst at 154-160lbs Jackson was an electrifying hitter, we didn't see what effect his punches had on heavier men. The other thing about Jackson is that for many people his signature KO is the one against Herol Graham. The point I will mention here is that no-one really knew how solid Graham's chin was, seeing as he was a defensive wizard for most of his career. It could be that Graham falls into the same category as RJJ; a man who was hardly ever hit clean but when he was he didn't have the mental fortitude to deal with hard shots to the head. I'm going to say Thomas Hearns, simply because he carried his power from 147 right up to 190lbs. Moreover because Hearns destroyed a truly great fighter with a truly great chin (Duran) whereas Jackson didn't even come close to this.
As a pure puncher as opposed to a great fighter Naseem Hamed warrants a mention here. He hit like a ****ing truck.
There was no chance of mental fortitude coming into play when Jackson hit Graham, he was out cold and I have little doubt that in the same circumstances that punch would knock out pretty much any fighter from 147-160. Whilst throwing a punch Graham was hit full force by a wound up right hook, he didn't have a chance. I agree about Tommy Hearns though, he showed at 175 that he still had power to be reckoned with. 30 pounds above his starting weight. He is #1 for me.
Thats a terrible way to go about it. His opposition was also poorer then the latter mentioned 2 and much of that KO percentage consists of journeymen and such.