A Clockwork Orange is a given, of course. But in addition to that (hopefully the images will work):
I admit this counts as a sign of my geekiness, but I really really like the soundtrack to Star Trek II: The Wrath of Khan. It's an amazing soundtrack, though I admit I might be biased due to the fact it's also one of my favourite movies.
Joe Hishaishi's soundtrack to Princess Mononoke is amazing. That tends to go to all of Hishaishi's work to be honest, but Mononoke is particularly flawless.
While I do not in fact own this, the soundtrack to Alex Proyas' 1998 film Dark City is pretty amazing. Or more to the point, the music from the movie (by Trevor Jones) is amazing. The soundtrack also includes "inspired by" tracks such as Gary Numan's atrociously bad "Dark" which don't work that well. But talking about the soundtrack itself this one is pretty damn good.