Ive been looking at this for 5 minutes working out how to answer straight..... I just cant do it, come on, a better question would be who hit harder at 47 Malignaggi or Mayweather?
Jackson had power that stopped a man in his tracks and had them knocked out before they hit the canvas. Roy Jones Jr, is a few notches below that.
Actually they had a common opponent Thomas Tate. Jackson went 12 hard rounds with Tate, Jones stopped him in just 2 rounds underrated win for Jones. Overall Jackson hits harder with one punch, but Jones is just as dangerous for his combination of speed & power, as the worst punches are the ones you don't see coming. Jones had some pretty scary KOs at Middleweight/Super Middleweight he hit very hard at these weights.
I think julian jackson might be the hardest hitting man p4p in boxing history. His power was so insane people seem to freeze up and drop down like they got hit by lightning. Just look at it. https://youtube.com/shorts/Gvb1ep37QLE?feature=share
At 160 and even 168 Jones had a ridiculous KO ratio, but for 1 punch power, you'd give it to Jackson. Hard to say as Jones was a slightly naturally bigger guy
Julian Jackson is arguably the hardest puncher pound for pound in boxing history so my answer to the question is him but Roy is an underrated puncher if he had the killer instinct of a guy like McClellan his KO percentage would be insane.
The Tate fight is an interesting point. Now Jackson was naturally smaller than Roy. By the Tate fight he was getting on in years and I think the retina issues had began by then. But he was still a very dangerous fighter and Tate was really in that fight. Thomas also went to war with Merqui Sosa, who was a very big puncher a division higher. Roy absolutely flattened him. And I believe is only man to stop him. So I do believe p4p it's Jackson no doubt but in real terms it's probably a lot closer than people are willing to admit. Roy around that time was beastly.