Simply put Ali had style, technique, and brains. Mike Tyson on the other hand was a brawler, who was very ferocious in his prime but lacked the technique and finese of Boxers such as Muhammad Ali, who truly made boxing into an artform. Mike Tyson was little more than pure aggression, and he'd lose the match if he couldn't find a way to end with within the first few rounds or so.
IF Tyson was able to get in fast and hard, he might take it. But Ali was so nimble, so quick on his feet, that I think he would run rings around Tyson and just wait till he dropped from exhaustion. One good punch, and pop, he's done.
Ali in his prime had the endurance to make a long fight work to his advantage, which is something Mike Tyson never really excelled at. He always tried to push for quick fights, by coming out swinging aggressively, which was highly effective, but he couldn't sustain it for longer than a few rounds. Once Tyson lost the ability to end fights fast, his career started heading downhill.