For me it's a measure of how well a fighter goes against a field of his peers. An example is I believe Dempsey achieved more than Bowe but Bowe does better against the notable heavyweights throughout history. Rating by h2h is a but more fun for me because it's usually reliant on watching fights of the boxers involved. Rating by greatness is more generally accepted but usually requires a lot more reading and trawling the internet. At present I rate mainly on h2h cos it's more enjoyable to do for me.