This is a relatively easy question to answer. There are 5 basic things that make a movie a classic.
1. Longevity- The movie must have survived the test of time. For a book, the test of time is around 500 years but for a movie, it is only about 10 years because movies are made and forgot very often.
2. Expediency- In a nutshell, the movie has to be worth watching. It must be full of good things like symbols and themes which make it worth discussing and watching over and over again.
3. Influential- When you watch the movie, you must be inspired to be or do something.
4. Universal Themes- The movie has to have recognized themes which are covered deeply so you can’t just watch the movie once and know everything about it.
5. Amazing Acting, Soundtrack, Cinematography, etc.- This is all the surface stuff that draws the watcher in to the deeper stuff.