Dont know if it would counts as a movie per se, but Interstella 5555 kicks some serious ass:
https://www.youtube.com/watch?v=bRt5z880CFY
https://www.youtube.com/watch?v=6S3ISlvlEbs
https://www.youtube.com/watch?v=HoQN7K6HdRw
https://www.youtube.com/watch?v=dKJfJMMsqX4
https://www.youtube.com/watch?v=Z6_ZNW1DACE
The only song I dont really "like" is Veridis Quo.

Another movie i liked because of the soundtrack was the remake of Dawn Of The Dead. Thanks to this movie I heard Johnny Cash and Richard Cheese for the first time:
https://www.youtube.com/watch?v=t91EkLt7mFo
https://www.youtube.com/watch?v=Tr8BgftEvBM

And the intro and the ending song of Dr. Strangelove:
https://www.youtube.com/watch?v=Qs7EikHQGlA
https://www.youtube.com/watch?v=wxrWz9XVvls

Many great picks in here. My favorite is probably Braveheart by James Horner.

I think my all time favorite is Apollo 13 (James Horner.)

And the rest of my top 10 in no particular order...
A Beautiful Mind (James Horner)
Titanic (James Horner)
Hook (John Williams)
The Shawshank Redemption (Thomas Newman)
The Green Mile (Thomas Newman)
American Beauty (Thomas Newman)
The 'Burbs (Jerry Goldsmith)
The Abyss (Alan Silvestri)
The Stand (W.G. Snuffy Walden - technically a miniseries, but oh well)

edit - man, I really didn't pay attention to the instructions of the thread. Oh well...

OH, you meant original scores rather than soundtracks. The I'll go "A Clockwork Orange".

When it comes to movies, IMO one of the most underrated movie composers was Basil Poledouris.

He got ROBBED for not winning with his Conan soundtrack.

He's composed some of (imo) the most iconic movie themes,
Robocop:
https://www.youtube.com/watch?v=D7rjLQuW2nQ
Conan:
https://www.youtube.com/watch?v=onGWF8mz1Zw
Starship Troopers:
https://www.youtube.com/watch?v=CIGHCoVzqtk

Also I think John Carpenter had some great scores for his films, the ones he did himself, like Assault on Precinct 13:
http://www.youtube.com/watch?v=RGME6jCwqw0

Such a memorable theme.

Ruthless BUMP for more or less the same reason.

Two soundtracks that have been on my mind recently:

Far From The Madding Crowd

I Heart Huckabees

I've noticed a couple of new films ripping from the Cloud Atlas soundtrack lately.

Some classics that first came to mind from the thread title:

Requiem for a Dream

Amelie

Harold and Maude

F for Fake

Also, on a side note it's interesting when music holds up a movie perfectly while watching it but can't stand apart from it. The most recent example where I experienced this was with Daft Punk's work on Tron. It's an amazing score for such a forgettably bad movie, but once you listen to it without the picture it lacks the action and drama the picture provides. The two are really inseparable. Interestingly the Glitch Mob remixed a track from it and the music video really gives you everything Tron had to offer in 5mins.

The Godzilla 98 soundtrack was really good outside of the greenday remix.

I'm a huge John Williams fan, I also like James Horner a lot or Basil Poledouris. Dave Arnold is great too.

Again, it's not easy to select my favorite one, most of their soundtracks have some catchy themes wher I think the earlier work of John Williams was much better than what he delivered during the last years. Then again, OST style changed drastically over the years, music in movies these days is not as important as it was in the 80s and 90s.

Literally anything by John Williams, James Horner or Hans Zimmer.
Legendary composers.

I prefer the original theme from the first Terminator movie, it is grittier and all, without "sad" choir vocals and whatnot.

It is a bit like the difference here:

This is the original Terminator theme:

https://www.youtube.com/watch?v=TmoeZHnOJKA#t=22

This is the watered-down version from Terminator 2:

https://www.youtube.com/watch?v=nJ-bIeJ4zOY

Once Upon a Time in America (1984) - music by Ennio Morricone

That would be Spawn OST.
The movie itself is just plain average. But the soundtrack is exceptional.

https://www.youtube.com/watch?v=mXYQQLBlyCE

The most innovative movie soundtrack ever.
Every track is a collaboration between two artists/bands with different genres, rock/metal and techno/dance.

It's still relevant to this day.