There are several things to consider here.

First: the eras. It's true that the 60's and 70's produced a slew of hall of fame heavyweights. Frazier, Foreman, Spinks (eww..), Chuvalo (misspell..), Liston, Patterson, etc. Muhammad Ali did defeat each of these fighters at least once. The difference between this era and the 80's (when Tyson peaked) is conditioning. The fighters of Ali's era were generally slow moving and would rapidly tire out. Frazier was one of few who could keep punching into the late rounds, and everybody saw how effective he was against Ali. Smokin' Joe really should have won their second meeting, but he was pretty much jobbed out of a decision. Anyway, Ali was able to take advantage of his slower opponents by throwing off their rhythm with jabs, and circling until they became dizzy or tired. If someone managed to get close, he would hug his opponents head until they were separated; which by the way, was a tactic that was later made illegal before Tyson even came on the scene. Muhammad Ali also made use of the ropes quite often, due to their looseness and flexibility. They (the ropes) were later tightened to prevent this from becoming a habit among heavyweights. By the time Tyson's era had rolled around, there were so many aspiring boxers that the pool simply became too large for any one man to dominate. Furthermore, the fighters now had to be prepared to strike with power and speed, as well as having the agility and stamina to go 12-15 rounds. The heavyweight scene had changed.

Second: styles. Joe Frazier was a volume power puncher. He could get in close and hurt you with body shots just enough to open your left side for a thundering hook. He was fairly one dimensional, however, as his right hand was no more than a parrying tool. Ali exposed this in the third fight by continually popping Joe over the right eye until Joe's already blind side had swelled completely shut. George Foreman swung with giant, tree-trunk like arms that were locked at the elbow. He usually had to throw several punches to hope to land one. While he was one of the hardest hitters of all time, he was notorious for tiring after 4 rounds or so. Given that he expected a short fight, and fought in 90 degree weather against a quicker opponent, it's no surprise that he was upset by the cunning Muhammad Ali. Again, Ali did use many tactics in this fight that were quickly ratified and would no longer be legal in any way, shape, or form.

Tyson is a different fighter than Joe or George. While his power is easily comperable to both of them, his speed was often overshadowed by his dynamite fists. He could bob and weave his way into a taller, quicker fighter's space, and chop them down with crunching combinations to the midsection. In fact, he even knocked a few fighters out this way. Often times, the taller fighter would have to block their ribs for fear of a break or internal bleeding, and Tyson would use this to land several clean head shots that put people out. Niether Joe nor George had the ability to fight with both hands as well as Tyson. Period. Ali NEVER faced a fighter with Tyson's explosive capabilities; in fact, Ali was even quoted as saying about Tyson, "...even I wouldn't be likely to handle him...". Ali depended on developing a steady jab rhythm against power punchers, and a fighter just couldn't get such a rhythm on Tyson. His head and body movement were usually enough to cut the ring off, no matter how slippery the fighter, and tear his opponents down. Had Cuz been there for the Douglas fight, Tyson would have had the greatest career in boxing history.

Finally: It's all theoretical. While these arguments sound neat and we can continue this "debate in a vacuum" forever, there's a good chance that we could all be very wrong about who would win. My conclusion: Tyson would win their first fight. If it advanced to a trilogy, Ali would eventually figure something out, because he was just that smart. Ali was the smartest fighter in history, and Tyson was the most explosive. Ali would win the trilogy, or at least tie it up at 1-1. If the latter is the case, then we'd set up the hearing. In the preliminary hearings of fight 3, Ali would taunt Tyson until he got ticked and ate Ali's bodyguards. Ali would sue Mike and send him to jail preventing a third bout.

Seriously, I think that Ali would probably take 2 out of 3 from any fighter; with Tyson being the toughest. I would guess that Tyson vs. Ali would be spectacular nonetheless.
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ali would win
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ronsaxton makes a good point.
i only compared Tyson to frazier because of the head movement. i couldn't compare Mike to foreman other than that the two may be the heaviest punchers in the history of the division. foreman stood and wailed hooks at the body with straight elbows. so differently than the place Mike generates his power from. frazier sort of had a take 3 to get in 1 style, making him unlike Mike.
Mike at his best could ko anyone.
i only hesitate to put his skills in this fantasy match up out of respect for Ali. Ali was/is a great human being.
i know this topic is not about that, but for me it is a factor.
Ali being the G.O.A.T goes beyond boxing.

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in all honesty i haven't seen enough of either of these two fight o really make a perfect guess. but the way i see it is even though ali experienced much stiffer competition in his prime he actually boxed ppl who were better than him in some ppl's eyes like frazier and foreman.

tyson however was swift and to the point, he simply didn't believe he could be hurt, and for much of his career that was the case, that is unitl he ran into buster douglas... he knocked everyone out and had power that i just can't believe anyone could have ever matched in ali's time.

obviously i know muhc more about tyson than ali, buti don know that ali was flawed in skill but very intelligent, tyson on the other hand was damn near perfect because u couldn't capatilize on his mistakes. and if you did it seemed to just make him madder. he was a freak.

i think in both of their primes w/ both of their gigantic ego's and b4 tyson went insane (literally, its pretty obvious he's not all there nemore, those punches apparantly did have an effect) i think tyson would win.
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I cant believe I didnt see this earlier. Ali would dismantle Tyson. Has anyone seen George Foreman fight in his prime? He hits just as hard as Tyson and he's a MUCH smarter fighter. He was dismantled by Ali. Who the hell has Tyson fought to say he's even a top 10 heavyweight??? Ali over Tyson in 6-8 rds, guaranteed!!
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