I have probably seen more horror movies than anyone else alive.
I love em !
From early stuff like Nosferatu, Frankenstein ,The Blob and the original Invasion of the Bodysnatchers, to the modern (often remakes of superior movies) horror of Saw, Hostel and of course the Silent Hill and Resident Evil spinoffs.
Some of my faves are the "bad" movies such as Braindead and Driller Killer.
All horror is to me is entertainment, with a large dose of dark humour preferably.
Ones to watch out for if you haven't seen them are :

Bad Taste
Any Toxic Avenger and most Troma movies (only if you are strange )
Evil Dead of course
Re-Animator ,, class movie


PS Aliens is a creature feature and therefore should fall under the catagory horror and it's only claim to sci-fi status is it's location.