Goodfellas or The Departed
I've seen each at least 10 tens, own them both. They're my two most watched movies. It does help that Goodfellas seems to be on TV at least once a week. Every time I see either one I'm still blown away by the different shots, the editing, the music and how it makes the scene flow so much better, the great stories being told (that were true or based on similar events), and the brilliant acting by each cast. There's so much to them and so much to what makes them good movies.
Sometimes I wish I could Eternal Sunshine of the Spotless Mind myself so that I could watch them for the very first time again.