Lennox Lewis - Lennox would have done well if he could avoid the big punch for the first 5 or 6 rounds, but that would have been very difficult and Lewis didn't have a good enough jaw to survive. If he makes it to the middle rounds, though, he probably wins.
Evander Holyfield - Holyfield in his prime was a cruiserweight - great fighter but never good enough defensively - loss by KO
Larry Holmes - Holmes in his prime was susceptible to an overhand right, and his instinct when he was hurt was to fight back right away and slug it out - loss by KO (but same as Lewis, if he makes it to the 8th or 9th rnd, he probably wins a decision
George Foreman - either way, it would not have gone more than 2 or 3 rounds. the Foreman who destroyed Joe Frazier was the hardest puncher in boxing history, so he was every bit as capable of KO'ing Tyson as vice versa. However, in his prime Tyson was so quick and so sharp with his punches, that Foreman would have been on the receiving end much more, so Tyson by KO.
Joe Frazier - two guys with almost identical styles. Tyson has more different punches though and his right hand is much more effective - Joe's ability to take the big punch was never that great, so Tyson by KO.
Rocky Marciano - The Rock - slow, too easy to hit, but could end any fight against any fighter with one right hand. Probably the second-hardest hitter ever behind Foreman. Able to take unbelievable amounts of punishment - never lost a pro fight - how could you choose against him? Well ,again, Tyson is just too quick and sharp with his punches, so I have to give him the nod. But a KO? I don't know. If Rocky won, it would be by KO.
Leon Spinks - give me a break. Leon had a prime? They would be taking him out on a stretcher.
Muhammad Ali - the 1967 Ali has no equal in ring history. (if you want to dispute that statement, first view the Cleveland Williams and Zora Foley fights then get back to me) A jab faster than Sugar Ray Robinson's, foot speed, hand speed, and reflexes never seen before or since in the HWT division. And don't think he was a weakling in the ring either - just look at his first fight with Liston, he was man-handling him in the clinches. Took one of the best punches of any heavyweight ever, but didn't need to back in the 60s because he almost never got hit with a clean shot. Tyson would have to score an early KO to have any chance, and that wouldn't happen. Ali in a late TKO after Tyson's eyes swell shut from the jabs and straight rights.