The soundtrack to No Country For Old Men was pretty interesting.

I still need to see No Country for Old Men.. I hope you mean interesting in a good way, haha.

So a score can use songs already written? Regardless, I'd say There Will Be Blood's score is my very favorite. If it counts.

Wiki quote:
'Unusual for a thriller, the Coens worked against Hollywood convention and minimized the score used in the film, leaving large sections devoid of music. The concept was Ethan's, who persuaded a skeptical Joel to go with that idea. There is some music in the movie, scored by the Coens' longtime composer, Carter Burwell, but after finding that "most musical instruments didn’t fit with the minimalist sound sculpture he had in mind [...] he used singing bowls, standing metal bells traditionally employed in Buddhist meditation practice that produce a sustained tone when rubbed." The movie contains a "mere" 16 minutes of music, with several of those in the end credits. The music in the trailer was called "Diabolic Clockwork" by Two Steps From Hell. Sound editing and effects were provided by another longtime Coens collaborator, Skip Lievsay, who used a mixture of emphatic sounds (gun shots) and ambient noise (engine noise, prairie winds) in the mix. The cattle gun used by Chigurh was in fact voiced by a pneumatic nail gun.'

That does sound interesting, haha. Now I'm excited to see it (to Netflix!)

This is my personal favorite.
The first part anyway.

After some thinking, I'm going to go with The Fountain. It includes: Clint Mansell (who also did the score for Requiem for a Dream), Mogwai, and Kronos Quartet.

Joe Hisaishi's score for Princess Mononoke FTW

The Piano, in my opinion, has the best score of any film I have ever seen/any score I have heard.

Il Buono, il Brutto, il Cattivo