My perspective on this topic is -- Consistency over Time.
Consistency = A players ability to dominate his over-all contemporaries of their time period. This does not refer to head to head battles, but the players ability to consistently win top tournaments against like competition. It is not necessary to hold the title of World Champion for this distinction.
Time = Length in years that the player was able to achieve this. The longer the better. So a time frame of 20 years is a good bench mark.
One of the endless debates is who is the best, or greatest. This is natural and happens across all endeavors. For our purposes we will stick with chess.
The problem with defining greatness is that it is dependent upon perspective and subjective judgements of those who apply them. So, how do we define the best/greatest at chess? In other words; what is the criteria you use to make this distinction? I am interested in seeing how differently we view this topic.
Please note that this topic is not about choosing who you think is best, but what method you utilize to come to that decision.