Impossible to pick just one! There have been so many great horror scores in so many different styles/creative approaches.
A few that I love:
Waxman: THE BRIDE OF FRANKENSTEIN Herrmann: PSYCHO Goldsmith: "The Invaders" episode of THE TWILIGHT ZONE Frankel: THE CURSE OF THE WEREWOLF Baxter: AN EVENING OF EDGAR ALLAN POE Sarde: GHOST STORY Young: HELLRAISER
The problem is distinguishing between true Horror and Giallo. I haven't seen most of these films.
Gli Orrori del Castello di Norimberga (1972) Stelvio Cipriani Lisa e Il Diavolo (1972) Carlo Savina Malenka, La Nipote del Vampiro (1969) Carlo Savina Il Sesso Della Strega (1973) Daniele Patucchi La Notte Dei Diavoli (1970) Giorgio Gaslini Il Conte Dracula (1970) Bruno Nicolai La Frusta E Il Corpo (1963) Carlo Rustichelli I Lunghi Capelli Della Morte (1965) Carlo Rustichelli La Terrificante Notte Del Demonio (1971) Alessandro Alessandroni Suspiria (1977) Goblin
Also Il Terzo Occhio / Un Angelo per Satana by Francesco de Masi and Il Mulino delle Donne di Pietra by Carlo Innocenzi which are unreleased.
John Williams Jaws without adoubt. Mark Ishams The Hitcher Charles Bernsteins A Nightmare on Elmstreet John Carpenters The Fog and Halloween Bernard Herrmanns Psycho
While I like (but not love) the horror genre, I'm generally not a big fan of its scores, finding much of the suspense music themeless. PSYCHO would be tops for me, at least in the film, even though it's still somewhat of a labor to listen to on its own. For listening away from the film, THE FURY is a winner.
Impossible to pick just one! There have been so many great horror scores in so many different styles/creative approaches.
A few that I love:
Waxman: THE BRIDE OF FRANKENSTEIN Herrmann: PSYCHO Goldsmith: "The Invaders" episode of THE TWILIGHT ZONE Frankel: THE CURSE OF THE WEREWOLF Baxter: AN EVENING OF EDGAR ALLAN POE Sarde: GHOST STORY Young: HELLRAISER
Without a doubt, THE HAUNTING (1963) composed by Humphrey Searle. My favorite horror film of all time due in large part to the score, especially in the opening prologue.
The Ghost of Frankenstein The Wolf Man House of Frankenstein The Satanic Rites of Dracula The Curse of Frankenstein Dracula (Hammer) The Final Conflict Night of the Living Dead Plan 9 from Outer Space The Thing (from Another World) The Fly (Shore)
Halloween (1978) Abbott & Costello Meet Frankenstein (although it's more fun/funny at times) Bram Stoker's Dracula The Omen (1976) Jaws The Thing (1982 Ennio Morricone) Sleepy Hollow Danny Elfman's The Wolfman The Theme to The Shining (the entire score is too weird for me, but that theme is awesome) The Haunting (1963)
I found a CD years ago entitled "Music From Scary Movies" by the 101 Strings Orchestra that had a lot of great stuff on it from various films. Has a tremendous, ominous recording of the Halloween Theme and the theme from The Shining. It gave me the idea of putting together my own CD to listen to. I ended up doing another that had some other themes/music from creepy films along with hold-overs from my original CD. I think I did a good job of mixing together some nice, creepy Halloween CDs. I listen to them throughout October. Some of it creeped out my kids as I drove them to school. Good times.
And I'll add Conti's THE FORMULA. 
