Comparing players is only made easier by the fact that modern technology allows us to view players everyweek all round the globe. Most people will nonly get to see players via the tv and as for players of old then a wealth of video, statistics, and other evidence is available in order for the more discerning person to offer up a more realistic comparison.
However, i feel it is important that fair comparisons can only be drawn relating to like for like players. ie forawrd type vs forward type. Also its important to realise that people tend to glorify players that appeal more to the kind of football we wish to see. Consequently a list of great players would probably contain more forwards than defenders. An interesting point of note would be that if you looked at the forwards im sure that a high percentage would have plied their trade in a team that boasted a miserly defence thus allowing them the freedom to express their talents.