Some additions:

The Crow

Trainspotting

Heavy Metal

O Brother, Where Art Thou?

That Thing You Do!

Walk the Line

Eddie and the Cruisers

Does Rattle and Hum Count?

I hadn't really realised it, but the entire soundtrack of Midnight Run is absolutely magnificent.

Seems appropriate given that its in a few days:

http://www.youtube.com/watch?v=fp9BSmxgUxs

But, really, it's one of my favourite main themes of a film, although that "number one" spot is shared with the likes of Blade Runner, Escape From New York, The Lord of the Rings and endlessly so on. However, if I were forced to pick a favourite then I'd most likely lean towards The Ecstasy of Gold by Ennio Morricone that I've seen posted already. Stellar music right there.

Some of the music used in Hitchcock films are really good too. Only see Vertigo posted so far but there's several tracks used in some of the films that are pretty amazing.

Wang Chung composed a great soundtrack to William Friedkin's brilliant To Live and Die in L.A..

http://www.youtube.com/watch?v=_XPy9CUAX80

Natural Born Killers simply awesome.

I Don't think anybody mentioned any Ghibli soundtracks so i have to vote for Nautica, Laputa and Mononoke combined : some really beautiful understated stuff. Anything with a very good Piano run or orchestral score gets in my book though...

I love the Drive soundtrack so much

you were always a hero to me Driver ;___;

1492 Conquest of Paradies

Not a movie but a series I loved in my childhood:
Robin of Sherwood

Blade Runner by Vangelis (Who also di 1492 BTW) and Dune (1984) by TOTO

Went back to Emerson's "Inferno" for Argento.

http://www.youtube.com/watch?v=3Qf52PXBKAc

and

http://www.youtube.com/watch?v=equGMj_6VjM

Cheesy but delicious, and more than worth the bump.

Boogie Nights

From Hell. Moody tunes composed for this the Jack the Ripper period costume drama, manna from heaven for a goth kid.

Crash. This is 1996 Cronenburg, not the showboating Paul Haggis message movie. Sparse, clinical, post-rock electric guitars plucking in an icy industrial landscape.

Lock, Stock, and Two Smoking Barrels and Snatch soundtracks because they're damn fun and clock full of classic and cult-classic British rock.

And any Nick Cave soundtrack collaborations (with his Bad Seed pal Warren Ellis), i.e. The Proposition, Assassination of Jesse James by the Coward Robert Ford, The Road, and Lawless. Anything that bard touches is gold.