Impossible to know. There have been plenty of brutal knockouts. However, it usually isn't about the punch being the hardest, just landed on the perfect spot, at the perfect time.
Yeah, thats where Tyson got the no socks thing from, he also tried to immitate Dempsey by not celebrating after a win early on in his career, though that changed in his second career.
Earnie Shavers right against Larry Holmes (at 2:10): [yt]PpNrzvCBWkc&feature=related[/yt] George Foremans uppercut on Cooney was immense, unfortunately the clips on youtube do not do it justice at all, I have seen some footage of it from a different angle and it looked absolutely lethal.
I think Chris Byrd gets the honors of being hit the hardest ever. When Ibeabuchi floored him is a classic killer punch. When Wlad floored him the first time in the second fight was also nasty. Both those punches sounded wicked.
Tua-Moorer was another brutal one. It doesn't look like quite as vicious on first glance because it hits him in the temple and not the jaw, but Moorer's response (or lack thereof) speaks volumes. Tua-Ruiz will also get a mention for good measure.v:deal [YT]BlS82LC3fTo[/YT] [YT]BLuV0vCWs_A[/YT] I thought Ruiz was dead after that last hook on the ropes.
It was an incredible shot, amazingly though it was NOT an instant KO. Willard got up and fought for a couple of rounds after that. It might be the hardest punch after which the guy kept fighting.