1. Ali-Based on competition and accomplishment I have to put him up here, not to mentioned at 1 time he was arguably the most famous person in the world, not just athlete.
2. Louis-26 title defenses when there was only 1 title. Significant impact within American society also.
3. Holyfield-For a 3 year period from 89-92 when he was the HW king he looked very impressive and damn near unbeatable. Had good speed and power and fought every Tom, Dick, and Harry out there.
4. Foreman-In his prime was a little too wild, but he is arguably the most powerful puncher in HW history, not to mentioned he became oldest HW champ in history during his 2nd act.
5. Marciano-Undeafeated for his career, was still undersized even for his era, but his heart and guts got him through, not to mentioned very good power.
6. Holmes-Very skilled fighter with an awesome jab, too bad he came after Ali and was never given his due by the wider general public.
7. Lennox Lewis- Very good skills, beat every man inside that ring he even avenge his losses. Has an impressive resume with good size.
8. Joe Frazier-Had a lot of hearts and guts and that left hook was brutal, reigned as HW champ for a number of years, only defeated by 2 other atg HWs.
9. Mike Tyson-For that 3 year period looked invincible, had power and speed for a HW and an iron chin, the guy had every tool except for height and reach, could have been up there with Ali and Louis, but his weak mentality was his downfall.
10. Dempsey-Had a very significant impact on boxing history, really made it popular in the US at the time and destroyed almost every man he faced in that ring during his reign with ferocious tenacity.

That's my list and my personal opinion. So if you disagree then so be it and move along, I just don't feel like debating about it, for these kinds of list are very subjective.