Lennox Lewis may be the best Head-to-Head Heavyweight fighter ever, as well as one of the most avoided. (Rid**** Bowe threw his heavyweight belt in the garbage, on national television, to avoid fighting him ) Prime Mike Tyson, it's hard to imagine anyone beating him on his best night. Ali and Holmes may be the most skilled ever. But lack Lewis's size, and Tyson's ferocity. Prime George Foreman. Before he fought Ali, he had an aura of invincibility about him that caused people to actually believe he might KILL Ali in the ring. I'd vote Lennox Lewis though. :deal
We have no idea how "Prime" Tyson would have fared against elite level opposition because he never fought any.
After watching some of Louis prime fights I think Lewis kicks Ali's ass. Great defense and a hell of a left hook, Ali's biggest weakness. He had amazing power in close. The amount of force he could generate throwing that left hook just a few inches is like nothing I've ever seen.