Marciano-Lewis would be a massacre. Lewis would have been just too big. Mine would be.. Tyson-Fraizer Mayweather-Cotto Morales-Marquez
01. Manny Pacquiao vs Oscar De La Hoya 02. Muhammad Ali vs Lennox Lewis 03. Kelly Pavlik vs Floyd Mayweather
So many ones to choose from...here's the top ones I've been thinking of lately: Dempsey-Liston SRL-Whitaker Tyson-Marciano Hagler-Monzon, Duran-Pryor, and Whitaker-Duran are top choices too, so I won't repeat them.
Ali vs Marciano (just so i can say to people how right i would have been that ali would beat hiom and stop him) Duran vs Chavez Leonard vs robinson (the battle of the sugar rays but more importantly the best fighters ever)
I like the three honourable mentions better than the three you've plumped for. Liston, Leonard and Tyson are all hot favourites in my view.
I think its odd how some people think Tyson vs Ali would be a "dream match" I think this would be very very one sided in Ali's favor.