I love top HW lists. My top 5 are always the same, but 5-20 change all the time. It's just too hard to figure out. One thing I used to do was include the original heavies like John L Sullivan and Jim Jeffries, but the sport has evolved so much that I really couldn't see them living with the super heavyeights of the last 15-20 years.

My Top 5 (not necessarily in order)

Ali
Louis
Foreman
Holmes
Tyson (Prime Tyson i.e a Tyson that any fight after Spinks is forgotten)

After that I would have the likes of Lewis, Frazier and Holyfield. Liston has question marks as does Marciano (woud've been mauled by a Super heavy of today)

If Vitali Klitchko had've been a round in an era where there were any real challengers could quite possibly of got close to a top ten list