I personally don't think Ali could have been beaten pre-Vietnam war saga. Joe Louis was also an amazingly gifted fighter. It's between those two for GOAT (greatest of all time) heavyweights in my opinion.
I also think Larry Holmes was very talented and underrated.
Both brilliant heavyweights that just went on too long. Louis was way too old to take on a Marciano at the peak of his powers, and the Ali fight against Holmes was a farce. Whoever organised that should have been lined up against a wall and shot. I don't think you can ever compare boxers from different eras and say who was "greatest". Joe would probably not have been big enough to be a heavyweight even in the late Ali era - and certainly not today. But skill and fighting heart? Unmatchless - if that's a word. :-)
What's the story behind the picture?