Maybe Tyson. I don't know, you could see Tyson slowly starting to lose it in the Bruno fight, but I think he was still very good at that point.
Mike certainly looked more lively going into that bout but I'm still skeptical. I don't think he was the same after 1988.. A major overhaul in management combined with some life changing events outside the ring and a loss of interest in the sport can certainly change a champion. There were some who even questioned his structural integrity after the Bruno fight.
Probably Douglas would win. You erroneously assume Tyson was prime for this fight, as you want to use it as Exhibit A that Tyson was prime when he fought Douglas. Your agenda is very transparent. Tyson didn't have to be prime to beat Williams. All he had to do was land his left hook. A way past it Mike Weaver knocked out Williams early with the left hook. Tyson's condition was in no way tested in the Williams fight. A better indicator of Tyson's pre Douglas form is seen in the Bruno fight when he was forced to fight for a significant number of rounds and his defense looked to have regressed already from the previous few years.
Its the Frank Bruno curse. Whenever Tyson beats Bruno, he normally wins his next fight in the first round, only to get tkod in the next fight.