Der...thought of a couple more right after I posted.
Blade and Blade 2 both sound great. Blade was (like ITMOM) available on DTS LD, but is DD only on DVD. I own the DTS LD and think it sounds fantastic. The DTS-ES track on Blade 2 sounds damn good.
Bride of Chucky had a great soundtrack and Dolby Digital mix. Not really a horror film, though. It's more comedy. Sleepy Hollow had a good Dolby track as well. I can't think of any real hardcore horror with truly great sound. Most of the great horror films are either low budget, Texas Chainsaw Massacre or older films. I'm interested in hearing some suggestions also. Check out www.horrordvds.com. They have some good detailed reviews.
So far, the best horror film isn't even on DVD, though it will be this August: The Haunting, Robert Wise's 1963 adaptation of the Shirley Jackson novel. It's in black and white and monophonic sound, and nothing could be scarier.
Well in that case I'd still recommend Alien, the sound design s subtle but still brilliant.
Fascinatingly the Legacy box set version of Alien (and posssibly the single disc as well I guess) has an isolated score track and as well as the original sound has a soundtrack comprised of on the set recordings. Creaky set floorboards and all!
I think that's probably unique isn't it? Or are there other DVD's with that feature?