Its impossible to actually put down one fighter because its so subjective. You could argue the case all day for the likes of Ali, Ray Robinson, Jimmy Wilde, Archie Moore, Henry Armstrong etc etc etc.
Personally I think HW's seem to get a leg up simply because they are heavyweights and at most other times aside from now it has been the most prominant division in the sport. I dont see anyone touting Joe Calzaghe on account of his unbeaten record which he is likely to keep but Marciano makes the list mainly due to being the only unbeaten HW champ. Since weight doesnt factor here where are Ricardo Lopez, Sven Ottke and Floyd Mayweather ? If being unbeaten makes the grade then surely they are up there.