In my opinion, it’s Garry Kasparov not because he was the best. In fact Magnus was the best, but you need to know that he was born with barely any good computers he was the best for over 20 years and was very dominant with Bobby Fisher he was by far more dominant than Garry but he was only world champion once and quit as a very young age
The thing is, with the chess engines nowadays, the margin for the best player is getting smaller. The fact that Magnus has been #1 for a good amount of time is actually pretty impressive, and if he’s still #1 after a few more years, he’ll be the undisputed goat for sure