To me a film soundtrack can be different things. Some are good to listen to even out of context of the film, such as the John Dunbar theme from Dances with Wolves, the theme song of Requiem For A Dream, Gollum's Song from Lord of the Rings or Jai Ho from Slumdog Millionaire. Some, like Hans Zimmer's Inception score and the score of American Beauty are good in the context of the film and what is happening on screen, and are less effective out of that context. I wouldn't really listen to these scores by themselves, though I think they're very effective in the film. It's hard for me to pick a favorite, but the score from the film Once would probably be my favourite simply because the music really elevates what would otherwise be a film with a fairly pedestrian plot and writing to be one of my favourite movies of last decade, and the music definitely fits the former category of film scores.
[youtube]http://www.youtube.com/watch?v=k8mtXwtapX4[/youtube]
[youtube]http://www.youtube.com/watch?v=0oyQPGJ5848[/youtube]
[youtube]http://www.youtube.com/watch?v=Db-2hPY0E5M[/youtube]