Young Lewis had amazing speed for his size. He could keep Foreman at bay, frustrate him for 6 - 7 rounds and then go for the kill. Same with young Vitali, prior to his shoulder surgery, when he still had one punch power. He had amazing reflexes and Foreman didn't have the style to beat a much taller fighter who uses his height well. I am not saying Lewis and Vitali are the best fighters ever, just stylistically, I feel the have the best advantages to keep Foreman from using his power and exploit his inability to adjust.
I agree prime Vitali and Lewis could give Foreman problems. Lennox could possibly do Well boxing for 6 or 7 rounds, until Foreman lands a flush straight right. As for Vitali I think he could do some damage, but Foreman's uppercut would finish him. Prime Foreman today would smash both klitschkos along with everybody else. Aside from Ali, I give Foreman a great shot against anybody. However, a prime motivated Riddick Bowe might be successful.
In his prime Foreman's sheer will and brutish style could neutralize superior technique. Gold medal olympic winner not too shabby.
Foreman wouldn't be the top fighter in any era after and including his own, or the one immediately preceding it because of Liston and young Ali.
Holmes, Tyson and Liston are all 50/50. I slightly favour Holmes. Holyfield of the mid 90's stands a good chance too, if he can survive the early storm. Lewis stops Foreman's wide looping hooks with straight punches right down the middle, that right hand will be landing on Foreman all night long.
Johnson is the blue print to beat Foreman. Big, fast, nimble, defensive phantom, durable. He'd block or duck Foreman's big swings and parry George's jab. In close, he would at be more-than-likely capable of holding his ground in clinches. I could see him taking a majority decision or late stoppage over 15. Ali would always beat Foreman. Louis, Liston, Bowe, and Holmes would all be 50/50. He beats all the HW swarmers quite handily with Dempsey standing the best chance out of all of them, which isn't saying much. He'd knock out Lewis and Holyfield.
Been saying this for awhile. Worse than Tyson post Spinks, or Liston post "I picked up a drink and became an alcoholic" whenever that was.