Something that should be mentioned in all of this is the factor of atmosphere as an important part in how scary such movies can be.
For example is how I found Aliens, Lord Of Illusions (a Clive Barker movie) and The Grudge (American version) scary and unsettling due to watching them at the cinemas.
The three factors that help the fear factor are the following:
1. That it's dark.
2. The surround sound.
3. Teenage girls screaming during the shock moments.
If there's anything more, feel free to add it.