It seems fairly obvious to me that the problem involved in this is selecting relevant, objective criteria. From previous posts, we see several different factors: longevity, success against contemporaries, real and assumed Elo ratings, the Transivity Law as discussed recently by WGM Pogonina, etc. I also suspect that not a not a small amount of national chauvenism might be involved as well.
The bottom line, IMO, is that it all has to be speculation at best, so I really fail to see the point of it all.
When Kasparov wins 20 games in a row against all GM competition , OR wins a candidates match with 100% score ( Fischer did it twice ! ) OR wins a major tournament with 100% score , only then will I ever believe that Fischer is not the greater player of the two. Fischer did ALL of these things AND did it without a whole army/chess school and govt helping him in both acceptable and unacceptable ways.......... nuff said. Fischer was more than 100 points higher rated than the #2 Spassky when they met in 1972 for their match. Was Kasparov ever 100 points higher rated than #2 even at his peak rating ? Fischer didnt benefit from ratings inflation as todays top players have. What was Karpov's top rating ? I believe it was 2780 and Fischers was 2785 ! I think there is no doubt that had Fischer continued playing he would have smashed the 2800 barrier and maybe even 2900 !