It’s a big sweeping question that can’t possibly have an answer. Is it the story? Is it the characters? Is it the cinematography? This examination by Now You See It attempts to break down what makes a movie elevate from good to great, and focuses on a few things: the impact it has on filmmaking (also known as why Citizen Kane is so great); the impact it has on our culture (also known as why Jaws is so great); and the impact it has on the individual viewer (also known as why whatever movie you think is great is so great).
A movie’s cinematic impact can forever change how movies are made, a movie’s cultural impact can change how we see the world, and a movie’s personal impact can help us process the life we’re living. So, I guess the answer on what makes a movie great is how much of an impact it had. I don’t disagree with that definition.