Aaahhh horror......my favorite genre. Let me see, where to start?
First of all, my all time favorite: The Exorcist
Then the following:
Top 5 old school
Nosferatu The Vampire (1979 Remake directed by Werner Herzog)
The Texas Chainsaw Massacre
Top 5 that came after
The Fly (1986 remake)
The Evil Dead (all 3 of them)
And many many others...awesome topic by the way.