First up who is "the greatest"?

Now this is a real tricky subject mainly because you always look at different weight divisions and then have to take a step back and think of the obvious like "Ali was a heavyweight!" which is not really comparable to say Roy Jones Jnr, who by all accounts fought at numerous weight divisions and was world champion at each of those weights to boot.
So the way I look at this along with many others is that you need the lb4lb ratings to confirm who really is "the greatest!" but recent or past greats are equally difficult to compare due to the way current promoters "protect" their fighters include this with nutrician, training techniques and even tactics that have all been developed and modernized. I mean if Ali were 22 years old now in 2009 and he was just starting out he truly would be the greatest of this era but people would always say "oh but you put him up against Frazier 30 - 40 years ago and he would have been beaten like a Dog!" and rightly so because that's the way we view heavyweight contenders now, like David Haye the British and world cruiserweight champ, he unified some belts and moved on up to heavyweight which is great for British Boxing since the heavyweight division has gone somewhat downhill of late and is mainly dominated by eastern European fighters who I might add are huge! But regardless of how good he is or becomes, Ali would always be the better in most if not all peoples eyes.

So all this in mind the lb4lb system is what we have to judge the best at the time provided they don't retire. And as far as "the greatest" goes I think we can only place them from an era of greatness, maybe just the decades in which those fighters dominated the sport. From that point of view Roy Jones Jnr dominated the nineties in the middelweight to light heavyweight divisions and was crowned lb4lb king for that. But I seem to remember that Oscar De La Hoya was also in the forerunners of that era, also crowned lb4lb king at a similar time in different weight classes to Jones Jnr except middleweight, I guess that the issue is probably just too difficult to piece together without the experts throwing every last piece of knowledge they have at you.

I will no doubt do a little more research and see what I can come up with, I'm certain many people will want to debate this as it's always a good subject.