It's hard to compare players of different eras. Instead, one should compare players against their opponents.
That being said, Fischer and his results in his run for the World Championshp helps to make him a candidate for the best ever.
6-0 vs Mark Taimanov
6-0 vs. Larsen
5 wins and 1 loss against Petrosian, in their match
7 wins and 2 losses against Spassky, in their match
These types of results haven't been seen before or since.
i think it's kasparov. after all, he was the one they chose to play against deep blue