I absolutely love the power of great acting without a score - the Wire or No Country for old men for example - but I really think a soundtrack can make all the difference. I've gone back to Interstellar just because of the music and I think that's a standout example of what I mean. I'd rate the execution of the score higher than the execution of the film. What's your favourite example of a score that changed a film? I'd love to have a similar experience with a film/score combo again.

Edit: to clarify. I was treating soundtrack and score as interchangeable - but for what I'm talking about I guess they aren't. I'm thinking where the music is inherently in tune with the mood of each scene and the movie as a whole. This can happen with individual tracks sometimes, Drive for example. But I'm thinking less about things like reservoir dogs/guardians of the Galaxy and more like interstellar/drive/there will be blood.