no kasparov
Who was/is the best GM of all time

I think Bobby Fischer. But I think you have to be rated something like 2600 to have a credible answer.

Also op, best at what? Like for insulting, zurab reaches standards set by korchnoi, nimzo and alekhine.

As time passes there are more and more people in this world and thus a larger potential for the lastest #1 to be the best of all time --up to that time.
Also as time passes the standard of living worldwide goes up and this would make a larger potential for more and more super good chess players and this also would give larger potential for the latest #1 to be best of all time--up to that time.
So, probably Carsen but Kasparov at his peak also very strong...

Morphy way down on the list as the population of the world was much smaller. Also the economic condition of the whole world was much worse and thus there were not that many good chess players...

I first thought that Kasparov was best but he cannot stand Carlsen and the new generation, as shown in the St. louis rapid

They have done research on this. 2 million positions and analyzed (1139 games from Carlsen) and similar for the others. They used SF with 3150 rating for the project. 1. Carlsen 2. Kramnik 3. Fisher 4. Kasparov 5. Anand
When you consider the greatest in any sport, and compensate for the advantages of modern times, I think you have to look at how much each player dominated his time. Bobby Fischer's domination of chess in the late 60's and early 70's was unprecedented and never repeated.
That kind of domination is extremely rare. It's like Babe Ruth hitting 60 home runs in 1927 when no other major league baseball team had a team total of 60 home runs or Wayne Gretzky scoring 92 NHL ice hockey goals in 1981-82.
morphy also dominated
When you consider the greatest in any sport, and compensate for the advantages of modern times, I think you have to look at how much each player dominated his time. Bobby Fischer's domination of chess in the late 60's and early 70's was unprecedented and never repeated.
That kind of domination is extremely rare. It's like Babe Ruth hitting 60 home runs in 1927 when no other major league baseball team had a team total of 60 home runs or Wayne Gretzky scoring 92 NHL ice hockey goals in 1981-82.
morphy also dominated
so it's either fischer or morphy
"The champions tournament held in 1948 to decide the next world champion ... was won by Mikhail Botvinnik with Vasily Smyslov in second place. ... The Soviet Union would, from that point until its dissolution in 1991, dominate the game at world level. Botvinnik went on ... to become the first Soviet world champion, the beginning of a continuous line of Soviet players to hold the title that was only broken for a few years in the 1970s by Bobby Fischer. ... What made the difference was the system of state sponsorship put in place by the Soviets to train and develop their chess players, ... selecting children ... who showed promise at a young age and sending them to specialist institutions where they were expected to follow strict and intensive training regimes ..." - The History of Chess in Fifty Moves by Bill Price
Is: Magnus.
Was: Bobby Fischer.