1) First off, a great movie should probably be a series of images projected at a succession rapid enough to convincingly approximate motion. The sounds of human speech, tires screeching, babies crying, dogs barking, and John Cassavetes exploding will sometimes be included, as well as music by Bernard Herrmann and the song "Night Shift" by The Commodores.
2) A great movie has at least one character with memorable hair. Doesn't matter what actor's body the hair is on or where the hair is on that body, it just has to be memorable.
3) A great movie either contains or at some point compels me for whatever reason to recall the line "I'm a fiend for mojitos."
4) A great movie will have Walter Brennan in it. If the movie was made after Walter Brennan's death, it will either have M. Emmet Walsh in it, or one of the rooms in the movie will have a corner where the outline of Brennan's ghost can be glimpsed, grinning or frowning at whatever is happening in a given scene.
5) A great movie has a party sequence of at least 30 minutes in length. The movie's greatness increases in direct proportion to the extent to which said party sequence lingers beyond strict narrative necessity, and the number of middle-aged Hungarians with accordions who are present.
6) A great movie will make me feel like I'm going insane as I watch it. There's some nuance to this one. If everyone else in the theater suddenly seems 15-20% more like a lizard than they did before the lights went down, great movie. If I start to hallucinate that the actors onscreen are bleeding from the eyes for no diegetic reason, I may only be watching a holiday-centric Garry Marshall production.
7) A great movie ends with someone cracking open a beer or a captive pigeon flying out a window or a woman we've never seen before roused from a daydream by a gunshot across the street.
Of course (as I then remind my interlocutor if they have not yet backed away slowly), my standards are not everyone's, but I like to think my formula has a degree of objectivity.