Dear Ahha,
It depends on what themes do you prefer. Pick the films you like the most.
I would say that for a good English, Films with a historical content or formal content would be better, for instance JFK, To Kill a Mockingbird, A Few Good Men.
Also pick films with an actor/actress that you think speaks clear and good English. I would have chosen Gregory Peck for English (for instance for French I like films with Michael Serrault).
Some films I personally like are Pulp Fiction, Forrest Gump, Goodfellas, The Matrix, The Usual Suspects, Dr. Strangelove, To Kill a Mockingbird, A Few Good Men.
I would suggest some general tips:
1.- Choose a film that you can/would like to see many times.
2.- A film that has close captions in English.
3.- See it without captions (use captions to check for what you didn't get) . Go back and forth as many times as needed.
4. Instead of a Film, you could do the same with books which also have an audio version.
Best regards,
Antonio