And hardest puncher is so hard to measure... Punch hardness would be to set up a machine like on Rocky, and have all the heavyweights hit it in their prime, and the highest reading would be the hardest punch....

I think in terms of boxing though, you could look at it in a much better way..

Who was the most devastating puncher? Who was the most capable of knocking someone out? Who translated their power into effective punching most often?

A guy could stand on a machine and swing and put up big numbers, but we can't tell what numbers each would have by watching fights.... You can however see who best used their power...

And Tyson was definitely up there in that respect. He made his punches count, and used what power he had to do the most damage... Some fighters have great power, but they'll be predictable with constant left hooks, and they aren't hugely effective. Even though they may have more power than other fighters...
Like Roy Jones probably didn't have the hardest punch on a measurement machine, but you add up the unpredictability of where the punch would come from, the lack of defense his opponent could use on it, and the times he would catch them with his hard rights - It resulted in a much more devastating punch that a fighter who swang away with all their power and didn't use it to as good effect..

That's my thoughts on the whole subject anyway when who has the "hardest" punch comes up...