Greatness, as said before is a very subjective test.
To my family, I am the greatest. But I suspect this applies to most.
To be the "best" at something is also an historical subjective test. As no one can account for changes in not just ability, but kit, training techniques, and rehab. Would the "greats" of yesteryear be even better, or worse, with the scientific breakthroughs that are available today. I'm not just talking about fell running either!
Also, would the "greats" of today have been as good without these breakthroughs in technology.
One thing that would stand is their ability. But who would have been the "best" "greatest". No one could ever say for definate. And why would we want to?
As far as I can remember there was only ever one sports person who proclaimed "I AM THE GREATEST". But even Mr Ali might now have to say, "I WAS THE GREATEST."