Who was the greatest chess player of all time and why?

Kasparov for me is the greatest chess player off all(he is also considered by many gm as the greatest). He dominated chess for 20 years against other great players like Anand and Topalov. Fischer did not proved himself that he could be dominant, he did not defended his title. Kasparov is like Pete Sampras ,he proved his greatness for long time, he is not a one hit wonder.