Whenever people aim to quantify greatness in any field, they usually opt for the Mount Rushmore example. "Who is on your Mount Rushmore of [insert profession here]." The exercise is a brilliant way to succinctly separate the wheat from the chaff by only selecting four prominent figures that exist at the apex of whatever field you happen to be discussing.
In the world of filmmaking, creating a Mount Rushmore of directors is virtually impossible. Some directors only play in a specific sandbox of one genre, whereas others can deftly maneuver between genres. Certain directors initially gain unassailable renown after a handful of classic films but suffer a steep decline in quality with their later filmography. Even with an exercise as subjective as selecting a Mount Rushmore, picking four great directors is an insanely difficult task. Well, at least choosing three out of four should be difficult because one slot has to be reserved for James Cameron.
In the video linked above, James Cameron sits down with GQ to talk about his titanic success in the medium of cinema.