I'm taking Nolan Ryan on this one.
I don't care if Mike Tyson puts his hands in front of his face, when he takes a 100mph fastball to the head and neck area followed by several more, over and over, he's going down.
And if Tyson does get up on him, I think Mr. Ryan could probably handle himself.
This isn't bat vs. knife. There is no way Ryan could stop Tyson. Not even if Ryan was in his prime and Tyson was who he is now. With the caveat that Tyson had to run in a straight line and could not duck, weave, or block the balls, Nolan might win.
If it were Tyson vs. Rollie FIngers, now that's a different story...
There's just no way Nolan Ryan could win.
At best, he might be able to get off one full speed fastball, but it's almost assured that Tyson could dodge it. Even if Tyson didn't totally avoid it, anything but a headshot probably wouldn't down him.