I don't know exactly what you mean by movie song, soundtrack or score? Soundtrack being auxillary artists' songs being put into the movie and score being music (typically symphonic) made for the movie. Either way, I have a few options.
My favorite movie soundtrack is probably
500 Days of Summer. Just a solid group of a some great alternative and indie music. Good film too if you haven't seen it.
My favorite movie score
BY FAR is the score for the recent film
Her by Spike Jonze. It's one of the most incredible pieces of music I have ever heard. Perfect for homework or rainy days. If you have 40 minutes, I highly recommend you listen to it through, it can bring in emotions you didn't even know you had. Again, the movie is unbelievably good too if you haven't seen it.