One of my favourite movie soundtracks (and movie) to date has to be monsters, directed by Gareth Edwards and composed by Jon Hopkins.
I was completely shocked by the movie itself, the trailer and movie title are deceptively misleading and I cannot think why or how this could have happened. I had watched it expecting a typical monster flick, but it was far from it. I later saw that someone had described it as Lost in Translation meets District 9 and I am inclined to agree, although I would say it leans strongly to the Lost In Translation side.
I was amazed by how good it was, the soundtrack repeatedly drops in at key moments in scenes and add that extra bit of emotional depth and atmosphere so much more effectively that many other movies manage.