This is a tough one for me. I'd have to put The Big Lebowski on top though. It's just a fantastic film, and I think it's amazing at bridging the real and the surreal. Plus it gives me an excuse to drink White Russians!
O Brother, Where Art Thou? is another favorite. The music alone is great. I listen to the soundtrack all the time. But the story, characters and actors are a tremendous combination. And somehow John Goodman manages to dominate his own scenes in the process.
Miller's Crossing was my own introduction to the Coen Brothers. I didn't even know who they were when this came out. But then I don't know if they were as well known as a directing team before Fargo came out. Anyhow, I would put Miller's Crossing up as one of the great gangster movies of all time. Perhaps not as epic as The Godfather or Goodfellas, but it seriously holds it's own against a lot of films in that genre.
I haven't seen No Country For Old Men yet, but I hope to soon. I've heard such great things about the movie, and I regret not having seen it in the theater.