If you want to help them practice their understanding of English through movies, I suggest you first show them animation movies such as the ones samuel suggested (Toy Story series, Ice age series, etc.). With a colorful visual support, it is always easier to understand the dialogues. If needed, there are always the subtitles you can use. I personally like the Shrek series a lot, as they incorporate humour and morals through an easy speech with an easy-to-understand story. The story of each movie (there are 4) refers to well known fairy tales characters as well. If you wanted to make short questionnaires on the movies, you could even refer on these. It would be appropriate if you wanted to include extra cultural information to your teaching! However, if you find good movies that are not animated and are easy to understand, go ahead. It is always enjoyable for students when they can understand the story. I think comedies are the easiest ones you could pick for your students' comprehension. Note that I never taught english as foreign language therefore I can only imagine what it is like! I hope I helped you!