Foreman, Liston & Tyson are the hardest punching ATG's. Big George took Frazier off the ground. That is something and Tyson didn't want any of it and that was against an old Foreman haha. I always wonder how Louis or Marciano would've handled Liston or Foreman. I can't see them winning in a slugfest.
In terms of one punch power, It would have to go with Tua, Lewis, then Foreman. Foreman didn't have many 1 punch knockouts but his immense stength and the sheer number of giant punches landed on you would put you out.