Tyson had fantastic in-fighting speed and aggression. His in-your-face style absolutely blew the opposition away, very much in the Frazier style of low body movement and explosive hands, until people twigged a way to frustrate him and stand off him. Would Ali have beaten him? - who knows, but it would have been one hell of a good fight!! Personally, I think Ali would have had Tyson, on the basis that he fought and eventually defeated similar fighters, e.g. Frazier & Norton. But they gave him big problems at their best.
As Gizmo says, we never really saw the best years of Ali. Angelo Dundee says that the Cleveland Williams fight was the best he ever saw Ali as a technician. He was improving on what was already a prodigious talent, but importantly, there were still some great boxers out there to hone his skills on. What fights they would have been!
A great athlete, boxer and a grand ambassador for sport.