Tyson lost every career defining fight he had, Ali was Ali, its not even a relevant question
That's a miss understanding of the type of fighter he was and the style he used. The Tevor Berbick fight was arguably a higher calibre fighter than he ever faced later on in his career, as was Spinks. He was actually the underdog in the Spinks fight and absolutely destroyed him
Tysons problems were always associated with fitness in his earlier days, not training etc. After then breaking off with Kevin Rooney, he lost that head movement. In the Bruno fight he was hit more times than he had been in any previous fight, and Bruno wasn't a high calibre opponent. That was pretty much the beginning of the end for Tyson.
If your style revolves around fast head movement, fast combinations, and sustained aggression; when you take mobility and fitness out of the equation you've lost your best tools. Tysons was never going to dominate for a long time due to his style, but his dominance was certainly cut short initially by his self destructive personality and later by his prison term.