Not to be disrespectful this is more to do with the facebook grief thieves, I love Ali, in and out the ring he became bigger than the sport..a true legend......but Sugar Ray Robinson was the greatest.
/\ yes but there are no cool reggae tunes about Sugar Ray Robinson
Public opinion usually favours the heavyweight. A good big one etc. That is why you have the notion of the best 'pound for pound' fighter. A sort of consolation prize.
That said, smaller fighters usually move around a lot more and are exciting to watch. The old three round ABA finals with Harry Carpenter commentating were great up until the light heavyweight division where we had truly hopeless types.
Undefeated carries a lot of weight, but you can achieve that by carefully selecting opponents or not fighting tough opposition.
Sugar Ray and Ali were both bigger than the sport. Sugar Ray was the first but Ali had more fame and impact.