Prelude and Fugue in C minor
I became obsessed with this piece (the prelude) at the age of 13-14, and started to practice. Of course, I never managed to learn the piece fully.
Since that, I have discovered it in some movies, and I just love the scenes they include this piece in. For example -
Le Silence de la Mer (2004). A young girl, Jeanne refuses to speak to the German officer who stays at her grandfather's house during the occupation of Paris in World War II. But eventually, there's a certain, silent sympathy between the two, and in one scene when the officer is leaving the house and Jeanne wants to warn him (you can see why in the movie, I don't want to spoil anything) she doesn't find any other way than starting to play the piano - Bach's prelude in C minor.
The Forsyte Saga - in one of the episodes there is a scene where the lovers Fleur and John have an argument and then, when John's mother can't stand the tension anymore, she starts playing Bach's prelude in C minor.
The Triplets of Belleville - a wonderful, French animated film, where the Prelude is one of the very themes of the film, variated in several ways (there's even a jazz version!).