It's Magnus hands down.
But before the computer era, some people worked harder (much harder) than magnus. They didn't have teams for preperation, most of them lived in unhealty conditions.. didn't earn thousands of dollars..
In this perspective i like old masters more. Like Nimzowitsch, Capablanca.. Who really added a lot to chess. Even Kasparov. Yeah i like him too 
There are many ways to measure a great chess player such as peak strength, longevity, rating dominance, and tournament dominance.
Peak strength: No other world champion dominated like Fischer was 1970-72. Kasparov and Carlsen are both close.
Longevity: Kasparov leads this category. From 1984 until 2005, Kasparov was ranked number 1 in the world. Carlsen has been ranked number 1 for over 10 years now and is still going strong so it's too early to tell.
Rating/tournament dominance: Fischer dominated most tournaments from late 60s to early 70s. Fischer was also a record 125 rating points above Spassky. Even though he won the 1972 match he still lost rating points. Kasparov and Carlsen are also close.
In my opinion, the greatest of all time is Fischer but Carlsen and Kasparov are also very acceptable answers.
Although, Carlsen is by far the best out of the three due to his access to information, engines, databases, and ideas that Fischer and Kasparov never had.
Magnus, kasparov, anand, etc in my opinion
Carlsen - Kasparov - Fischer - Capablanca - Lasker
http://www.chessmetrics.com/cm/CM2/PeakList.asp
Carlsen's longest winning streak is 6 games in a row. Kasparov 7 Karpov 6 Fischer 21
Magnus
Nihalsarin is de goat he won against hikaru
If not then magnus carlesen
I am the GOAT, because I really like eatinG OAT, am G O AT the top, and I always GO AT it.
My chess GOAT is Viswanathan Anand. He is my idol. Put your thoughts as well on who is your GOAT of Chess