Who is the best chess player ever?

And yet Carlsen is fighting hard to get to 2900 FIDE Classical, a feat never before accomplished by a mere human.
Theoretically, there is no limit to the rating one can attain, but breaking 2900 is a truly daunting task for even the most skilled players in history.
3000 FIDE Classical is at best a far distant, fanciful dream even for the likes of Carlsen.

Magnus Carlsen. AT present we have the best top player pool in history. And Carlsen still dominate for such a long period above all these best top players.

A good argument could be made for Vishy. A virtuoso who excelled (excels) at all time controls, five-time world champion, came up in a country that wasn't a major chess hub at the time (didn't have the advantage of those who grew up in Soviet/post-Soviet states), gargantuan influence popularizing chess in India, and got close to winning the championship in 1998, which would have been the toughest championship win ever probably (an exhausted Vishy versus a rested Karpov).

Whoever it is is the strongest
Deep Mind stopped developing Alpha Zero so the strongest version of Stockfich would beat Alpha Zero nowadays.

Gioachino Greco was the originator. All who follow imitate. Greco was the original master, the first chess professional, the GOAT.