To this day, I believe George Foreman is
the hardest hitting heavyweight boxer. The biggest difference is
how he throws his punches. Foreman, especially when he was older threw punches that looked like it didn't have a lot behind them. It honestly looked almost like he was throwing borderline arm punches. When you watch Ngannou or Wilder you see them throw their punches with explosion, there is significant speed behind them, they throw it hard and fast to the point where their technique has been criticized. When you watch them strike, you know those punches are powerful.
With Foreman, however, you see him knock out opponents and you get confused, it didn't seem like the punch he threw had anything behind them yet his opponent is slumped over, unconscious.
You see it here with Ngannou. He lunges forward, throwing wide hooks, putting everything behind them:
You compare that to this iconic knockout from Foreman. The sweat that turns into mist when Foreman hits Moorer with a one two doesn't coincide with the very little windup Foreman generated:
It's also very hard to find GIFS that don't result in finishes, but Foreman's performance against Holyfield also showed his insane power. He hurt Holyfield numerous times in their fight with punches that seemed like they were thrown half-assedly. Holyfied even has this somewhat famous story of him asking his corner if Foreman had knocked out all of his teeth after he got hit by him
And when he did decide to plant his feet and throw heavier punches, he still barely had any windup on his punches at all. He chambers them and lets them go.