Here's my first take on it: 1. Chuvalo 2. McCall 3. Ali 4. Baer 5. Tua 6. Mercer 7. Marciano 8. Holyfield 9. Foreman 10. Dempsey 11. Jeffries 12. Cobb 13. Holmes 14. Uczudun 15. Quarry 16. Bowe 17. Liston 18. Young 19. Toney 20. Marion Wilson
good list. I think everyone should reconsider Jimmy Braddock. He had a hell of a good chin fought some big punchers, including Max Baer, and was only floored in once in 86 fights (Louis), many of them fought under nurished and on short notice). The beating he took for 8 rounds from Louis should be enough to put him that list. I would add Tyson as well.
My best recollection - and it's been twenty years or so, and this isn't exactly a fight one reads up on! - is that there was some after the fact assertions by Cobb that he either a) took a dive; b) was on drugs; or both. I remember being rather unconvinced and believing that Cobb was basically making after-the-fact albis for his getting knocked out by some tomato can. All of that said, the guy had a hell of a chin, and there is no question that this KO is atypical of his punch resistance. But still, it does justify his lower standing, imo. I might also add that Cobb's era - the early 1980s - isn't exactly one of the best in HW history, either.