I tend not to use music when I've actually been playing; on the other hand, I inevitably do so when writing or designing; mood music is almost inevitable for the creative process. I also tend to be the kind of person who can listen to the same song on "repeat" for hours on end (thankfully, my wife is also that kind and our musical tastes are pretty compatible, so no one gets a stuffed penguin tossed at them

).
With that said, my favorite Gothic-horror mood music includes:
The soundtrack from
Bram Stoker's Dracula--One thing I exceptionally like about this soundtrack is that it has music for a variety of different scene types: "Love Remembered" and "Mina/Dracula" for romantic scenes, "Vampire Hunters" and "The Brides" for generic Gothic-adventure moments, "The Storm" and "The Hunt Builds" for action scenes (on a side note, I always thought the scene where "The Hunt Builds" plays in the film is especially well-done for the use of music combined with cinema images...)
The soundtrack from
Castlevania: Symphony of the Night--Guardian of Twilight is right about this one. The techno pieces aren't particularly Ravenloft ("Tragedy of a Young Prince," for example, is a great song, but the electric guitar doesn't do it for me as RL music), but "Prayer," "Requiem for the Gods," "Nocturne," "Woodcarving Palteeta," "Waltz of the Pearls," and "Lost Painting" are all exceptional.
Beethoven's "Moonlight Sonata"
The soundtrack from
Shadow Hearts: I mentioned this in a similar thread about a week ago, but this soundtrack contains some of the best Gothic-horror mood music I've ever experienced. "Coffin fetish" has become something of a theme song for reading the works of Basil Copper.
Other videogame soundtracks:
Castlevania: Lament of Innocence
Koudelka
American McGee's Alice
Koudelka
Devil May Cry (Track 9 on Disc 2, "EV-18," is very atmospheric)
Anime soundtracks:
Vampire Princess Miyu (both the OVA and TV series)
Lunar Legend Tsukihime
Fate/stay night (okay, so I like Kenji Kawai's music for horror

)