Tyson's combo of speed and power is unmatched, so I would put him as the harder hitter. There are people with HEAVIER hands/punches but they don't have the speed which IMO negates them as the hardest hitters because speed adds that extra oomph.
No order:
Tyson
Shavers
Foreman
Hearns
Manny, yes, Pac
Duran
A few others



how i forget those


Foreman was still competitive with the elites of the 90's he even had Evander Holyfield roiding ass, hurt a few times, too bad Tyson was too p*ssy to catch that fade