There is no greatest of all time,just the fighter you happen to like the most,at any given time.
All these fighters fought in different eras,weight classes,which presented their own set of benefits and serious challenges.
Obviously the matchmaking system is much looser in the past,and the stance towards PED's. But these men would fight twice,sometimes three times as much as the average fighter now..for alot less pay. Alot more short notice fights,alot more last min opponent changes. Sometimes you didnt even know what the full card was until you watched it. Alot less ways to study your opponents because footage on them would be difficult,and sometimes impossible to obtain. Pride didnt even have public weigh ins!
Those people with their noses up in the air about how much better (or more talented the fighters are) it is now,ought to think about that.
Now things are presented alot clearer,with a rankings system,that helps to keep things competitive and more difficult (for some) to make it to the top. Would todays stars have an easier time in the wilder more loose way the sport was run 20 yrs ago? Or would the hardened warriors who didnt mind throwin hands 6 times a year,dominate the current crop of guys we have,and make them look like sissies?
There's no way to tell. So shut up.
You could say Khabib was the best LW of his era,and that BJ was the best in his. Thats about as far as you can really go.