Teofilio Stevenson at one time was supposed to fight Ali but it never came about. He would rate high though.
Dempsy and Marciano were both small by todays heavyweight standard and Archie Moore was a ligt-heavy but it ain't so much the size of the dog in the fight as the size of the fight in the dog.
Sam Langford may have been the best ever pound for pound as he was at best a middleweight soaking wet, yet fought and won against the worlds best heavies. Nobody would give him a title fight due partly to fear and largly to racism.
http://www.iabpff.org/regions/site/sam1.html
Liston should not be on the list as he was a Mafia patsy with a string of fixed fights.