1. Ali 2. Louis 3. Marciano 4. Tyson 5. Lewis 3 and 4 are interchangable, head to head Tyson is in the top two beating out Louis but factoring in resume and best wins he's behind. I have Lewis at 5 above Holmes and Liston but they could just as easily have been there, but Lewis reign as champion and resume is superior to Liston and head to head is probably ahead of Holmes.
Tyson should rank above Dempsey, on any list. Head to Head he was a bigger, stronger, faster, more powerful, and more skilled than Dempsey. Basically a Dempsey hybrid. Resume wise he also has a MUCH more impressive title reign and beat many more contenders than Dempsey and often times more convincingly. There's no way Dempsey should rank above Tyson, he's worse in pretty much every way I can think of except foot speed and this is coming from a Dempsey fan.
Good lord, Lewis is RIDICULOUSLY overrated on this site. He was the ONLY top level HW champ taken out twice in the early rounds by second rate fighters when he held the title, and he didn't fight any of the other three best figthers of his era when they were even CLOSE to their best. He's borderline top ten - at best. My list: 1. Ali 2. Louis 3. Dempsey 4. Foreman 5. Holmes
My Top 10: 1. Louis 2. Ali 3. Holmes 4. Lewis 5. Marciano 6. Liston 7. Frazier 8. Tyson 9. Johnson 10. Holyfield