Other ways of asking about it are: どんな映画が好き？ 何の映画が好き？どの映画が好き？ It's like "what kind of movies do you like? / what are your favorite movies?". I hope this helps you. Chao

「一番好きな映画は何？」　is translated into English as follows: What is the most favorite movie? So in this case, you can choose one movie. On the other hand,however, 「好きな映画は何？」 is translated English as follows: What is your favorite movies? So in this case, you can choose whatever movies you like and of course it's not necessarily the one movie,but it's ok to choose several movies you like.