Although, I don't agree, there are no shortage of the great boxing men who'd still pick Joe over anyone else, I say still because many are dead. Ray Arcel actually thought Dempsey was still the best, this was in the early 90's, I love Jack Dempsey but I don't think his style would do well at all in later eras. Ray did though and would say the dempsey that beat willard would beat anyone else. All subjective. I say Ali in the mid sixties was the best, not just because of all his other abilities but moreso because of his amazing stamina, Marciano was the only heavyweight who could throw as many punches as Ali. It's still a basic sport, a guy who can hit the other guy more, usually wins, and in that department, ali was able to throw and land extremely well and take few punches in return in that era.
Holyfield is another fighter who I think might surprise a lot of us on his best nights he was a handful.