Well, I do think Isaac Newton.
whats considered classical ?

To give a somewhat straight answer, there is probably no one player or time that defined the transition from "classical" to modern chess. In his 5-volume series of modern chess history, Kasparov begins with Wilhelm Steinitz, world champion 1886-1894, and works up to modern times with Vladimir Kramnik. Each added insight and style to the game. I'm sure others have alternate views.
When I think of classical I think of before Botvinnik time since he sorta invented the concept of opening prep