Realistically it would have to be the all time great heavyweight matchups,

Ali vs Tyson
Dempsey vs Marciano
Jack Johnson vs Joe Lewis
Foreman vs Lennox Lewis

Then the winners play in a semi, then a final.

My money would be on an Ali vs Jack Johnson final with Johnson winning by stoppage in the 25th round becuase if it's all time rules no way I'm letting these modern day pussies get away with 12 round girlie fights!