I consider a film "great" or "of the best" when...
The film's story is interesting and original, and is well assisted by all of the film's elements. Instead of trying to have shallow DOF, "cool" color grading, and awesome camera shots, using the look and sound of the film to subtly push the idea of mood, time, location, and tone of the story. One example is Raging Bull. The film has very harsh, high contrast, and sharp black and white images to show the bleakness and pain of the film's characters. In Hunger, McQueen brilliantly used silence to show the power, grimness, beauty, pain, and emotion of the situation the characters are caught in.
When a film can capture something amazing and large within an everyday task or seemingly mundane task or action. In Sofia Coppola's Somewhere, she took something very small and turned it into something large by creating interesting and engaging characters. Same goes with Lost in Translation.
When a film stays with you. When an image, sound, or is engraved into your mind, and it doesn't leave for a while. In the masterpiece The Shining, Kubrick used horrifying and strange images to create mood and push the intensity of the story. The bear and the waiter, Jack's icy face, the twins, etc. A more recent film would be Gravity. Sound design was used to great effect into the film. It made the film more real and lively. I was disappointed when the film ended. It felt like just a few minutes. The director had created interesting characters and an engaging plot, and used all of the cast, crew, and gear to it's fullest. The intense sound and remarkable visuals made the film all the more amazing. I can still hear and see what happened. The movie is still in my mind.
Most importantly... a movie should entertain you.
Movies are a form of entertainment. What's the point of watching a movie if you're bored out of your mind or don't give care about any of the characters. You should enjoy a movie, not dread the experience. All of my favorite films entertain or at least interest me. The Lives of Others and Ben X were flat out amazing. I was on the edge of my seat and with the characters.
But of course, everything I have written so far is purely my opinion. I'm sure someone else could disagree and say something 100% different.
There is no "best" film. No film can be ranked. No form of art can be ranked.
