I think Jack Johnson may have to say something about that. For me it´s Moore. Of course guys like Hopkins, Duran, Fitzsimmon are to be considered. In general I think hws reach their peak a bit later and can fight for a bit longer than the lower weights. That needs to be considered too.
I won't argue with Moore if thats the consensus. After 40 Moore fought around 30 times picking up a light heavy world title in 1957 and losing by TKO in the 4th to a young Ali in his last but one fight. Duran fought around 26 more fights, losing quite a few including loses to Camacho (twice), Vinny Paz (twice) and a rising Joppy. Its probably fair to say that Foreman achieved more after 40 than the others and also fought in more relevant fights against better opposition. He fought about 20 times after 40 but he also had the ten year lay off. Foreman fought at top level facing the likes of Holyfield, Briggs, Moorer and Morrison. He also won the HW title against legit opposition. Foreman also faced faded guys like Cooney, Bert Cooper and Alex Stewart who had reasonable records of their own. Foremans achievemets, consistancy and quality of opposition is better than Moore or Duran. Respect to them both but its Foreman.
I'm sure I read in some book somewhere in my past that there were stories of a 17/18 year old Liston being hired as a sparring partner for Two Ton Tony Galento when he was training for Joe Louis - does anyone else ever recall reading this anywhere - if anyone knows the book that would be good as I can't remember which one it was in - might have been Dark Lights??
In adition to the names mentioned, I will suggest one less conventional pick. Jem Mace was 39 when he unified the British and American titles to become the first world champion, and most of his defences of that title took place in his 40s. He fought Mike Donovan to a 4 round draw in his 60s and many observers thought that he had the better of the matter.