Well, if we're limiting ourselves to originally composed music for a film, then that makes this easier for me:
THE MAGNIFICENT SEVEN - Elmer Bernstein
Bernstein is a very overlooked composer. He was a true workhorse composer like Goldsmith, Morricone, and Hermann, but for some reason, his sound is so easily confused for the work of others that it's hard to place exactly WHICh ones are his without iMDB. Either way, he was damn, damn good.
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ALIENS (Bishop's Countdown) - James Horner
Funnily enough, Horner's most recognizable track isn't the main theme of the film, but rather a 3:00 cue from the film. Ironically for that, it's more well known for its use in just about every action movie trailer from 1986-1999. Either way, it gives you real shivers and rushes every time you hear it.
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JOHN CARPENTER'S THE THING (Humanity II) - Ennio Morricone
Simple, repetitive, and undeniably creepy as fuck, John Carpenter's musical hand is fairly intrusive on Morricone's, but the fact remains that this is probably Carpenter's richest musical score, and therefore, mostly Morricone's.
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HELLRAISER - Christopher Young
Christopher Young is the greatest living composer of horror film scores and the Hellraiser score is his unconquerable masterpiece. The entire score is based on the macabre variations of waltz rather than traditional tropes of the genre, and the result is a haunting, beautiful miniature symphony that wouldn't be out of place in a drama.
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JAWS; STAR WARS; SUPERMAN - John Williams
With Williams, you're never sure where to start. Star Wars is the most obvious choice (which is why it's here, along with Superman), but Jaws is perhaps his most iconic thanks to that brain-meltingly simple 2-note chord that sets the stage for everything that follows it. While 3 may seem too much or too little for such a prolific composer, the Superman theme--the one below actually being the "Alternate March" version--in my opinion, supplants Aaron Copland's "Fanfare for the Common Man" as the theme for the hero in all of us.
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TAXI DRIVER - Bernard Herrmann
Of all of Herrmann's legendary catalog of work--Citizen Kane, Vertigo, North By Northwest--Psycho is undoubtedly his most famous. But I feel that theme relies a bit too much on orchestrated sound effects rather than actual music. But his final score--literally the last thing he did before dying hours afterward--is a lush, mercurial love song to the schizoid emotional romances of the title character borne from the city he inhabits. An elegant combination of militaristic snare and languorous jazz sax, Hermann had never been so smooth.
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BULLITT - Lalo Schifrin
What can you say about Lalo Schifrin that hasn't been shouted across the room at a rival gang of Henry Mancini fans before a bottle fight broke out? The man, like Mancini, is one of those impossibly distinct composers who created a sound that, thanks to the monoculture of the 70s, will never die nor escape the permanent association with said decade. That said, Bullitt was written in 1968, but the groundwork was laid for the next 10 years and the ensuing 30 and onward with the classic opening sequence and all of its snazzy, jazzy, and burpy brass swagger.
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THE CLASH OF THE TITANS - Laurence Rosenthal
Rosenthal isn't as well known as he should be. He was a classic standby-studio man from the Old Days of Hollywood, known for scoring epics like Becket (1964). But what I always remembered was his name on The Clash of the Titans (1981) because I HAD TO KNOW who wrote that adrenaline-pumping theme song with all of its old-school charm, whimsy, and excitement. It's for that reason that I've selected that very track for this example.
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POLTERGEIST; THE OMEN; LEVIATHAN - Jerry Goldsmith
I've saved this for last because it's a damn tough one. How do you find the right theme song to illustrate the oeuvre of such a dynamic and diverse composer as Goldsmith, a man who vies with Williams as the man who put music to the hopes, dreams, and nightmares of the 20th century? Do you go with Alien, which doesn't so have so much of a theme as a sonic introduction? What about Star Trek: The Motion Picture, which became the official theme of The Next Generation in 1987? The Mummy? Chinatown? (The man scored Chinatown for fuck's sake!) But my final selections came from the distillation of Goldsmith's nature as a composer: a narrative-driving theme that never shook the prevalent undercurrent of mood that the entire movie was about. Poltergeist captures all of the play-by-play momentum of the plot but always lacing it with the current of innocence and hope with the nursery-rhyme-like leitmotif of Carol Anne's Theme. By contrast, The Omen does the same with the relentless menace and sinister intent of the powers that created and protected the Antichrist always lurking behind the swells and silences of the cues--IMHO, no other film yet made has managed to surpass The Omen as "scariest fucking soundtrack of all goddamn time." And to cap off the Goldsmith demo is the magnificent closing title credit theme for the 1989 B-movie Leviathan, a surprisingly powerful, rousing, and undeniably auteurist score for what should have been a rote, glib, and bland paycheck project. I guess Goldsmith was just plain incapable of doing a bad job.
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Honorable Mention: PROMETHEUS - Mark Streitenfeld
The theme to Ridley Scott's Prometheus is called "Life" which certainly seems apropos for this main title track that builds an ominous suite of escalating mystery and promise as the movie opens on an alien landscape that is about to become a hotbed of life.
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