I dont know. I wouldnt say its Ali, he is the self proclaimed gratest and people bought into his own hype. You havent included some of the more recent fighters either, De La Hoya and Mayweather are regarded by many as the best pound for pound boxer of all time, although I wouldn't include Mayweather as for me personally "The Greatest" needs to be a reasonably decent person as well and Mayweather has a history of beating up women.
Before seeing this I was just reading about Anthony Joshua. Although its way too early to consider him for this accolade yet, he is certainly in contention for the future. Still fairly young and with 18 KO's out of 18 fights and an Olympic gold medal winner to boot. A world champion with an average of less than 3 rounds per fight. If he can keep his win record going, let alone his KO record he could be in with a shout in a few years time. And add to that that he is from North London and had a history of being arrested a few times and he has turned his life around and is now the epitome of clean living, publicly at least. Reminds me a bit of Rocky :lol: