People tend to forget how much technique Tyson had back in the Cus days. He has the power, but he also has the head movement, quick feet, etc to go along with it. He could box as well as punch. It's a fact people overlook.
As far as Ali vs. Tyson, I'm going with Tyson.
However, that's only if Cus & Rooney are with him. Without those dudes you seen what happened to Tyson. Cus was his rock, his father figure & when he died Tyson just...lost his edge I suppose.