Zefarelly's right: most of these are just complilation albums rather than genuine soundtracks (and no-one's mentioned The Harder they Come or Natural Born Killiers, two of my favourites in this category).
Best 'real' soundtrack album is Curtis Mayfield's Superfly. Pure class.
Bloody hell - these are nearly all compilation albums, probably because they're cheaper to put together than paying a composer to write original music. (Oh, Gwylan's already alluded to this...)
Anyway, my ORIGINAL soundtracks are
The Good, The Bad & The Ugly - Ennio Morricone (superb trumpet solo for the three-way showdown at the end)
Once Upon a Time in America - Ennio Morricone (Said by many critics as THE soundtrack by which all others are judged)
Once Upon a Time in the West - Ennion Morricone (simply AMAZING main theme)
A Fistful of Dynamite - Ennio Morricone (simply AMAZING main theme)
Le Professionel - Ennio Morricone
Superfly - Curtis Mayfield
Shaft - Isaac Hayes
I am not particuarly a fan of Westerns, but when it comes to soundtracks composers, Ennio Morricone is quite the simply the greatest ever, beating some pretty stiff competition such as John Barry and Lalo Schifrin.