The real hypothetical question is who would win a Chess960 match? A 29-year-old Fischer, Carlsen, or Kasparov.
I'm American and I believe the top three greatest chess players are Carlsen, Kasparov, and Fischer. I think it's hard to compare the three as they all come from different eras in chess data that was available. I think if they were all in their mental primes today with the same information available I don't know if I would be able to pick who I would expect to win a typical classical format tournament. Maybe I might favor Kasparov slightly if at his peak of performance?I think only Americans would ask this question, rest of the world ask Kasparov vs Carlsen.