Very hard and demanding question with no definitive answer,i presume people are going to focus on the big names such as ,Bobby Fischer and Garry Kasparov but keep in mind their are others just as good who haven't or didn't see enough of the spotlight,btw i have no answer,a good analogy for this question would be,which religion is real?.You are going to get a flow of different answers for different reasons.
Who was/is the best GM of all time

Borislav Ivanov was the greatest ever.
Yes!
That's who the Houdini program gets all its moves from!!

Borislav Ivanov was the greatest ever.
Nope, he never even earned the GM title. He never won a top flight tournament, even with the use of an engine.

GM Jesus. He never went pro, but it was as if God Himself was playing
Too confusing. First they said he got help from his dad. Then they said he was his dad. No wonder the bishops hung him from the towers and all the king's 8 pawns and 2 horsies could not put him back together again. And then he came back down, and the horses never walked straight again.

Borislav Ivanov was the greatest ever.
Nope, he never even earned the GM title. He never won a top flight tournament, even with the use of an engine.
They wouldn't let him. He could've been a candidate. He could've been somebody. Instead he became a dope dealer.

to be precise not all listed here are GMs.
https://en.wikipedia.org/wiki/Grandmaster_(chess)
"Since FIDE did not award the Grandmaster title posthumously, world-class players who died prior to 1950, including World Champions Steinitz, Lasker, Capablanca, and Alekhine, never received the title"
It's just too difficult to pick the best of all time in chess. Some athletes you can tell they would do well and be one of the best or one of the best in the modern game. Ex, Jack Nicklaus, Wayne Gretzky. But Chess I have no idea who would be better. If anything it's more likely to be a series of draws before somebody wins one game.