Who is the GOAT of chess?

Depends on how you define "greatest" and "best".

Best right now is Gukesh.

Greatest IMO is Carlsen.

I think a case could also be made for:

Kasparov:

Kasparov received a Chess Oscar eleven times as the best chess player of the year, in 1982–1983, 1985–1988, 1995–1996, 1999, and 2001–2002.

Between 1981 and 1991, he won or tied for first place in every tournament he entered.

With the exception of the PCA period and sharing first place with Kramnik in 1997, Kasparov led the rating list from 1985 to 2006 – a total of 255 months.

Karpov:

Karpov's chess tournament successes include over 160 first-place finishes.

Held the world number one ranking for 102 months, the third-longest all-time behind Magnus Carlsen and Garry Kasparov.

Jose Capablanca:

Capablanca had an impressive undefeated streak from February 10, 1916, to March 21, 1924.

This streak included a period where he was unbeaten for 9 years and 283 days.

Out of 571 games in his adult career (1909-1939), he lost only 34 games.

Between 1914 and his World Championship match against Alekhine, he lost only 4 games in 158 match and tournament games.