There is a comparison, even if Kasparov wins it. IMO the best place to start would be the highest tournament performance ratings[1]. Kasparov had five of the top ten at the time the list was compiled, over a ten year span. That's pretty dominant. OTOH, Carlsen's later 8/10 at Pearl Springs 2009 (Category XXI) got him a new record PR of 3002[2]. If we include matches, his current performance against Anand might also rate pretty high.

I'm not saying Carlsen is the greatest ever. I'm just saying comparisons are totally valid.

[1] http://en.chessbase.com/post/the-greatest-che-player-of-all-... [2] http://www.chessgames.com/perl/chessplayer?pid=52948