I am a huge horror fan so I have a fair few to name:
The original Dawn of the Dead is a flawless look at a zombie apocalypse, and a cryptic message of consumerism which sadly came true, as evident today...
The Orphanage is a masterpiece Spanish horror by Del Toro, would recommend it any day of the week
The Nightmare on Elm Street movies
The scream quadrilogy, all flawless
Drag Me To Hell and Insidious both scare the hell out of me every time...
Quite a few more!