strongest players to never becomes world champions!

Well, I think turn of the century players are probably less well-known than mid-20th century Soviet players by now - due to the passage of time.

But all the books that I enjoy feature players like Tarrasch, Nimzovich, Rubinstein, Marshall, Pillsbury, Flohr, etc.

World championship contenders. Especially Rubinstein - who I don't think had the opportunity to play for a WC match due to World War II - if I recall correctly.

#31
No, Rubinstein tried to play a world championship match before World War II, but he could not raise the money, the same story as Nimzovich. Starting around 1930 Rubinstein suffered from mental problems and he was admitted into a mental asylum. Nimzovich died from pneumonia in 1935.