I think there are three ways of looking at "Who is the best player ever":
1) Purely on highest rating. So Magnus Carlsen then.
2) How much better was the player than his peers. Fischer and Lasker might be two real candidates here, some might say Morphy and Alekhine too.
3) How long was the player the best for. Easily, Kasparov.
It is natural that Carlsen should respect Kasparov, as the only living rival for best player ever.
he seems to have deep respect for Garry