Well, here's some food for thought about Sugar Ray Robinson. He won the world middleweight title 5 times, and he LOST it 5 times too. If he was pound-for-pound better than Ali, what about the five guys who beat him?
Answer - there's not really such a thing as an 'unbeatable' fighter - for goodness sake, Ali came within seconds of losing to Henry Cooper back in 64 or whenever it was - only Angelo Dundee 'accidentally' ripping his glove bought him enough time to recover from the 'Ammer'.
Henry Armstrong - there's a name to conjure with. Held world titles at 3 different weights (when there were only 9 to choose from - not the current 20 or so and 5 versions of each) - a good candidate.
Roberto Duran? A great fighter from a great era, but why is it only welters or middleweights who are considered to be 'pound for pound' better than Ali?
Jack Dempsey and Rocky Marciano must have a claim from the heavyweight division, such a shame that Teofilo Stevenson never turned pro too.
Like cricketers, though, you can't compare between eras. Mind you, boxers today are puffs. Couldn't go 75 rounds to save their lives - and what's these things on their hands? Gloves? Why's that - a bit cold?