Those that could beat Ali are those that can best match him speed wise and perhaps certain swarmers. Johnson Dempsey Tunney Louis Marciano I do not believe Holmes, Lewis or Tyson would beat prime Ali.
In the second fight Liston was intimidated by Ali. He lay down. Not intimidated in a "You hit me with your hardest shot and I'll do the same" kind of way......Liston was fine with that kind of scenario. It was a different sort of intimidation - Sonny was scared of being made to look a bigger mug than he was in their first bout. Similar to Duran in the second Leonard fight,really.
Holmes, with Eddie Futch rather than Ritchie Giachetti in his corner. Dempsey, fast on the close with his body shots and power from both sides. Foreman is a question mark. Ali did not take on that proficient a ring cutter during the 1960's, and we don't know that he had the same level of punch resistance. Would the legs he had against Williams, Liston and Terrell be sufficiently energetic to keep him out of harm's way, or would George have still forced him to engage as quickly as he did in Kinshasa? Nobody I'd definitely pick over him. He's still the fastest HW of all time at his peak, both with hand and foot speed, speed kills, and he proved his championship distance stamina with Chuvalo and Terrell in the 60's.
None of those decisions were even controversial in my book when I watched them. Mind you, I was the biggest Ali hater back in those days. Spinks beat a overweight fatass imitating Ali on suspicious circumstances via split decision. You won't however, mention who won the rematch. Don't worry, I'll save you the trouble. Ali won the rematch, with the judges giving Spinks no more than four rounds. :good
Of course Ali beat the amateurish, alcohol-addled Spinks by decision in the rematch. So? Even Ali haters in those days didn't make much of a fuss about the questionable decisions rendered in Ali's favor, probably as a result of being conditioned into believing that he was the best. In hindsight, without the propaganda and hype, his record doesn't reflect reality.
To pick the best fighters to beat Ali, you have to know his weakness and stylistics tendencies. Ali had trouble with good jabbers, and very aggressive power punching types with a chin and stamina. You could insert Kenny Norton, but there are better picks. Vitali Klitschko Holmes Lewis
I would not. Tyson is a front-runner type, and mentally did not perform well when faced with adversity.