Fischer, Karpov, Kasparov, and Capablanca are without question the top 4.
The only fair way to judge the best in any sport is how good was he against the best of his era.
From 1914-24 Capa NEVER LOST A GAME.
Fischer blew through the 1971 Zonal against some of the best GM's of his era. Timminov 6-0! Bent Larsen 6-0! Against Tigran Petrosian he caught a cold it was close, and then ran 4 straight on him. Then won the title. Then quit.
Karpov from 75-85 was just the most dominant player of his time. I loved Fischer as a kid, but let's face it, he was scared of Karpov.
Kasparov, was the best player for 20 years against GM's. He beat Karpov in his prime when he was only 22 and Karpov was in his prime. Then the whole FIDE thing got in the way. NO doubt it was 30% his fault, but the FIDE President of that era was the worst.
So you would have to give the nod to Kasparov.
Morphy, was early chess. He was great no doubt, but a lot of his brilliant games were against players who were NOT GM level.
Chess players reach their peak at 30, Fischer vs Kasparov both at 30 who would win?? It would be one for the ages!! Both like Capa, were artist.
emm......