Some great ones already mentioned.
As I'm a huge fan of gialli and Italian horror cinema, I'm also a big Goblin/Simonetti fan, particularly when it comes to the Argento scores. Two other favourites in regards to the genre are Morricone and Cipriani, both of whom have produced some beautiful scores.
Badalamenti's
Twin Peaks compositions are also a work of genius, and I'm another who's a fan of both
The Wicker Man and
Natural Born Killers soundtracks.
Clint Mansell is another I want to mention, with his brilliant work for Darren Aronofsky on composing the scores to many of his films; his scores for
Pi,
Requiem for a Dream, and
The Fountain being my personal favourites.