Tua hit harder than Foreman. Reason being, I think Ali would not have been able to Rope-a-Dope him; he just hits way too hard.. (Although Ali would have danced his pants off a la Lennox Lewis)

My list:

Tuaman
Shavers
Foreman
Tyson
Marciano
..... honorable mention ....
V Klitschko

At any weight hardest hitter
Gatti
Marciano (if u consider him a LHW)
Hearns