Dividing fighters in to weight divisions is an artificial situation.
We don't use age as a way of determining greatest, so why weight?
For example, there is a Canadian marathon runner who ran under 3 hrs for distance - in his 70s! Now that is phenominal, but nobody is going to call him one of the greatest runners of all time.
No, pound-for-pound maybe a fun thing to talk about but the heavy-weights are the greatest fighters because they would beat all the best from other divisions.
Think of Ray Robinson in the mid-50s. Well he might have been great at middle-weight but a young Floyd Paterson would have flattened him out in no time.