No one will ever know.
Unless we put every boxer that ever lived on that sport science program and test their punching power.

Even then, thats an experiment and wont take into consideration the adrenaline, fear and confidence you have in the ring to give you that extra bit of edge to push those punches.

Heavyweights will always hit the hardest but if you look at it from a pound for pound status who hit the hardest i would say a toss up of hitman hearns, julian jackson, nazeem hamed (sort off!).

Ali said earnie was the hardest hitter he faced so those words alone put him right up there when you look at who ali faced..