Re: best heavyweight under the old rules ???
Well, by nature a larger man will always find a distance fight tougher, so I think it goes without saying.
An Ali, a Frazier, Holyfield, Marciano...these guys could match up with anyone from any era as far as endurance goes. Some of the old-time fights (like pre-1920) weren't fought at any great pace; certainly not the pace of fighters from the 30's onwards.
What fascinates me is the conditions the old-timers fought in.
Going way back, the Sullivan v Kirain fight took place in the heat of the day and lasted three hours. At the end, Sullivan was badly burnt from the sun, and Kilrain was no doubt severely deydrated since he had been chugging down whiskey in between rounds.
That's some kind of tough.