Who is the best chess player of all time
When I'm asked that question about any sport, I consider it in terms of who stood farthest above the crowd. For example, in American baseball, the greatest home run achievement was Babe Ruth's 60 home runs in 1927, a year when NO other team's 25 players hit 60 home runs combined. Since then, players have hit 70+ home runs (playing 8 more games/year) in more perfect lighting, etc. conditions, but several teams are exceeding 200 total home runs each year. Babe Ruth dominated as no other has since.
In chess, Bobby Fischer's amazing run up to the World Championship in 1972 involved an unbelievable crushing of three other top-ten players to reach the title match and a year before that of total domination.
In 1970, against mostly the strongest players in the world, Bobby won 48, drew 23, and lost 3, including 2 wins, 2 draws, 0 losses vs 1966-69 World Champ Tigran Petrosian.
In 1971, in the candidates knockout matches, he won all 6 of his games against Taimanov, all 6 of his games against Larsen, then he was ill while playing Petrosian and "only" won 5, drew 3, and lost 1. The championship match was won by Bobby 12 1/2 - 8 1/2 where Spassky got one point for a game in which Bobby didn't show up. Bobby played for draws after he got a comfortable lead.
No one has ever dominated like that. So, if Bobby was given today's advancements in training, knowledge, information technology, etc., I would expect he would dominate now. Of course, he'd still be a jerk and maybe slightly insane, but he'd be the best in chess.