For the moment I only want to share some thoughts on method:
I believe that "greatness" consists mainly of three components:
- Relative strength compared to one's own generation of competitors. I will call this factor 'R'.
- Absolute strength (the quality of moves when the complexity of positions is taken into account). I will call this factor 'A'.
- Potential strength (estimated strength if given the same conditions as players from other epochs or areas). It should be noted that this is not exactly the same as talent, as work ethic is an important ingredient in potential strength. I will call this factor 'P'.
I suppose the main question is how these factors should be weighted and combined into one final ranking. For starters you could just give all three the same weight, simply to see whether the result looks reasonable. Then I suppose you must decide whether to add the numbers or multiply them. Probably the results for the top contenders will not be too different however you choose to do it, but instinctively I prefer multiplication (if for no other reason, because I called them 'factors').