Does anybody remember what Crowley said about magick? The proper application of the proper force, in the proper amount at the proper time? Punching power is a lot like that, in that it doesn't matter how hard you can hit, if you can't hit the other guy when he is in a position -"ready"- to be hit.
Henry Cooper, all 185 pounds of him, did a test long ago where they measured the force of his left hook, after it had traveled 5 inches- 5 inches!!-and the force was rated to be equal to a 16 pound bowling ball hitting the headpin. The point being that any trained fighter, a guy that spends all day every day practicing hitting things, can kill you with a punch. But sitting down, winding up and smashing on is a bunch different than hitting a guy that doesn't want to get it and has also spent all his time practicing hitting you.
That is why I think Joe Louis is the most effective HW puncher of all time; he could pull the trigger and finish you with either hand at any time. I think that ought to be the measure- effectiveness versus 'hardest' because that is basically irrelevant.
By the way, that cut in the Lewis/K fight, came from a glancing thumb. K rolls that punch off his shoulder as Lewis is really turning his fist, it catches K with the thumb area.