My top three possibles: 1. Joe Louis. 2. Rocky Marciano. I know. I know. I am going with Rocky Marciano as the 2nd most likely to beat Muhammad Ali anyway. 3. Evander Holyfield.
Rock, paper, scissors. A slugger beats a swarmer because the swarmer comes to him. A boxer beats a slugger because he picks him or her apart .A swarmer beats a boxer because he doesn't like to be crowded. Of course there are exceptions. The only boxer to beat Ali when he was closest to his prime was a swarmer and he spent three weeks in the hospital afterwards. Tyson or Marciano are going to be brutal affairs.