2 of my favourites of all time were Demon Knight, and when the whole rap/metal crossover thing was a brand new concept, the Judgement Night soundtrack.
Man, showing my age now!!
More recently, Driver had a pretty sweet soundtrack.
Judgement Night was one of the first ones I thought of too!
Other top soundtracks are:
Pulp Fiction
Trainspotting
Human Traffic
Local Hero
Tron Legacy
Bill and Ted's Bogus Journey
Highlander (an entire earth where everybody listens to nothing but Queen! Even back in the 18th century!)
Pi
Heat
Side Note: Do you know what's funny? As I type this 'shuffle' has just fired up Kenny Loggins' 'Highway to the Danger Zone' from Top Gun!
Platoon
Natural Born Killers
Easy Rider
The Beach
Vanilla Sky
I'd love to say Apocalypse Now but it cuts too much off the Doors classic "The End". Dammit!
And purely for the theme tunes:
Das Boot
The Dambusters
688 Squadron
The Good, The Bad and The Ugly
The Great Escape
Bridge on the River Kwai
Enter the Dragon
Bullitt
I can't choose a favourite so I'm going to go all hipster and say the best soundtrack I've ever heard was Asian Dub Foundation doing a live soundtrack to classic french film 'La Haine' at a screening in the Sidney Myer Music Bowl. Shame the twats who organised it screwed it up by using white subtitles for the film. It's in black and white ferf***'s sake. Couldn't understand a damn thing!