This has been discussed ad nauseam.
You can not compare different eras, even if conditions had stayed the same (like 90s vs 80s), much less when conditions have changed that much (current era vs any previous era).
My take is that, even when conditions stayed basically the same, for example 90s vs 80s, it is senseless to say that Sampras was "better/greater" than Lendl or McEnroe, because 10 years apart is too much, they are from different eras, so nobody has a clue about how many great tournaments+YE nš1 would McEnroe or Lendl have, had they been born 10 years later, or had Sampras been born 10 years earlier.
The current era is even more problematic because for the first time in tennis history, all tournaments conditions are quite similar, medium to medium-slow, and it obviously produces a very low number of different GS winners (basically the best two or three players of the moment) and because of that those "top-players" numbers will be clearly inflated (the argument is called "the Decathlon example" as I have stated many times).
So almost the only sensible thing to say is that X player was the best or most successful player of Y era, Z player was the best or most successful player of W era, maybe C player was the second best or second most successful player of H era,.....
For example, Federer has been the most successful player in the current era (last decade or so), Nadal the second most successful, Djokovic the third (and being able to order them is possible only because they play in the same era). Or for example Sampras was the most successful player of the 90s, Agassi the second most successful, Courier the third.....
And even that simple thing is tricky, because sometimes an "era" is not well defined at all and is totally subjective.
The most funny thing about all this is that Sampras fans hated me ten to fifteen years ago when I told them this sensible view of things, and now Federer fans will hate me because of the very same reason (not all, in fact I know for a fact that many Federer fans actually agree with me wholeheartedly, some of them from these forums as well).
Last edited by mattennis; 02-11-2013 at 09:26 AM.