I don't really see the point in making a top 10 argument here, but I think you are trying to see two points as being mutually exclusive. You say that Tyson was exposed by better fighters (which happened to come along in the 90's) and by virtue of this his defense which was at its best in the 1980's, must therefore have been flawed. The truth is that his defense was THAT GOOD. It was exceptional. He was really hard to hit cleanly. You take away that active defense, and you get the plodder we all know when he stopped training as studiously. Had he trained as feverishly in the 90's as he did in the 80's you would have a very interesting fighter. One that might have had to fight 3 fights with Holyfield to split the difference. I'm not saying a peak Tyson would walk through Holyfield or Lewis, but I am saying that the guys he fought after his prime had the advantage of beating a guy that wasn't at his best. Hey, listen, Tunney, beat Dempsey, but also, Dempsey was three years rusted and getting up there for an active pressure fighter. These things happen.
It's important to recognize that in Tyson's prime he had exceptional defense, and when he fought top tier guys later on, he wasn't at his best anymore.