As Carlsen has said, Fischer was the most dominant player ever. In the early '70s, long before ratings inflation, Fischer had a ELO rating of 2785 when Spassky, the world champion and second highest rated player, had an ELO rating of 2645, and Petrosian, the third highest rated player and former world champion, had a rating of 2625. I don't think that anyone else has ever come close to that ratings gap over the rest of the world. From late 1970, in the Palma interzonal tournament to select the candidates for the world championship match against Spassky, through late 1971, in the final candidates match against Petrosian, Fischer won twenty straight games against world-class competition, including his last seven games in the interzonal, six straight games against Taimanov, six straight games against Larsen, and the first match game against Petrosian. (A 20-game win streak against world class competition is, as far I know, unequaled, and there are few instances of anything close to that.) Then, after a loss and three draws, Fischer reeled off another four straight wins to finish off Petrosian, who was generally considered the toughest player in the world to beat.
Finally, in the world championship match against Spassky, after losing the first game and forfeiting the second because of his dispute with match organizers, he won seven games to Spassky's one, with a final result of 12.5-8.5 (including the one-point forfeit, which did not count for ratings purposes, and a bunch of draws after the match was effectively decided).
That said, there's a big debate about the "best" player ever. Although Fischer was the most dominant, his serious playing career came to an end before he was thirty. Kasparov, in contrast, was the best player in the world (by most accounts) for a much longer time. (I don't recall how long; I'm doing this from memory, and there was a long period when I simply didn't follow international chess, which was in a fairly chaotic state for a while, at least at the world championship level.)
The debate over the "best" ever is generally between Fischer and Kasparov supporters. (Ventolin seems to favor Karpov, but that's a rather small minority position.) Some old-timers would cast their vote for Jose Capablanca, who was almost unbeatable from about 1914 to 1927, and some would go all the way back to Paul Morphy during the 1860s. Of course, some now will say Carlsen, although he's clearly not as dominant (yet) as Fischer, and hasn't (yet) had Kasparov's career.
For what it's worth, when Carlsen has been asked about the best ever, he has said that Fischer was the most dominant, but Kasparov the greatest in light of his much longer career.