[sc_rufctr]

At their peak and with cage fighting rules I'd say Bruce Lee but by boxing rules, Ali.

What about Mike Tyson versus Muhammad Ali? (Both at their peaks) Mike Tyson would have taken Ali apart IMO.

If you get a chance you should watch the "Thrilla in Manila". Fight three, Muhammad Ali vs Joe Frazier.
Joe lost but he punished Ali like no one ever did before. By the end of the 14th rounds Ali could barely stand & he collapsed after the decision was made.
They stopped the fight because Joe's managers thought he was near death.

Truly epic. I doubt we'll see a fight like that ever again!