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The Exorcist has been played out to the point that it's a comedy, but the scene where she is walking down the stairs turned upside down.... holy **** man.
 
Some have already been mentioned

The Exoricst (1973)
The Conjuring
The Ring (US)
The Shining
Poltergeist (1982) - at the time, now not so much
The Blair Witch Project
The Sixth Sense
 
Some have already been mentioned

The Exoricst (1973)
The Conjuring
The Ring (US)
The Shining
Poltergeist (1982) - at the time, now not so much
The Blair Witch Project
The Sixth Sense

The Ring and The Grudge were good because they harkened back to the days when horror relied on suspense, not brute force. Their Japanese roots also gave them a different feel.
 
its hard to give just one answer. here are some of my choices:

insidious
the thing
jeepers creepers
devils rejects
hostel
texas chainsaw massacre
dawn of the dead
 
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