I know I'm in the minority here. In fact I would have laughed at myself years back for even considering such a comparison. But after watching and analyzing damn near every boxing fight I can dig up tape on, my beliefs change over time.
When I compare boxers who never could fight one another I compare them both in their prime. Usually a window of time during their career. This window needs to span long enough to make sure that they aren't a fluke. Neither of these fighters were flukes. So I'm not comparing Tyson later in his career who was outboxed and classed by Holyfield and Lewis. By this point his physical attributes he relied so heavily on had atrophied from age and drug use. I'm also not comparing Ali who got dominated and knocked out by Larry Holmes and pushed into retirement by Trevor Berbick.
These two in their prime I picture.
Tyson was pro at 18 won his first 19 fights by knockout, 15 in the first round. 26 of his first 28 wins by knockout. Tyson beat Trevor Berbick (same Berbick that beat Ali late in his career) to win the title 2nd round KO. He was the youngest heavyweight champ of all time and the first heavyweight to unify the titles. When he beat Spinks to unify the titles he KO'ed him in 91 seconds. He is the only person to knockout larry Holmes (same Holmes that knocked out Ali late in his career). At one point he was 35-0 with 32 wins by way of knockout.
That's a lot of crushed cans bro.
I watched this man in his window of time absolutely demolish everyone in a fashion I've never seen anyone come close to. Overwhelming physical domination. We all know his infamous downward spiral starting with buster Douglas loss and imprisonment but in his prime he was capable of beating anyone and was arguably the hardest hitter and one of the fastest punchers of all time.
If Ali couldn't handle Frazier's power or Ken Nortons, or even Spinks i think he loses to Tysons. Ali lost to all 3 of these guys in his prime window. Only fight Tyson lost in his prime window was Buster Douglas. Ali's early losses set up some great, historic rematches that we all remember but I don't see him surviving Tyson until later rounds and picking him apart. Maybe in their second or third fight he'd figure him out like he did with Norton and Frazier. He was lucky to get the decision against Norton in their second fight after his first knockout loss to him.