Re: Heavyweight Champions--Who would be the best in MMA?
Tough to say, because we don't know how they would adapt to grappling.
I'd favour the strong, rugged fighters over those who boxed and moved. Guys like Frazier, Marciano, Jeffries, Tyson, maybe Sullivan, maybe Liston...those types of fighters. They have a lower centre of gravity than the tall fighters like Lewis, Ali and Klitschko and that surely could only help defending against takedowns and that sort of thing.
I think they'd make naturally better wrestlers too. I can only assume a Jeffries, Frazier or Marciano would have a more natural aptitude for grappling than an Ali or Holmes, based on strength and more of a willingness to fight in the trenches.
That's my 2c anyway.