I don't know if he was the very best but David Bronstein would have won the World Championship match against Botvinnik had he not been nobbled by the KGB and told to lose. Bronstein was a genius but inconsistant, Keres was very strong and consistant,but for longevity and strength you might give it to Korchnoi. The greatest? I would say Tal for his brilliant play over a long period of time when he had a severe health problem!
anand, i think.
there are of course better players, but they are world champion, so...