I'm a sucker for cheesy 80's slasher films. Especially the low-budget, relatively unknown ones - The House On Sorrority Row, Mirror Mirror, My Bloody Valentine, The Funhouse, Sleepaway Camp, Happy Birthday To, The Burning Me etc. But in terms of seriously scary films that I love, I think the "Alien" series is pretty spot on. It's been a LONG time since a film actually scared me, but the suspense in Alien and the jumps in Aliens get me every time. Also a big fan of the "Nightmare On Elm Street" and "Halloween" films, but who isn't?
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I prefer the classics, but recent horror flicks worthy of a mention include Wolf Creek, Creep and The Hills Have Eyes remake. Very few people make decent scary movies anymore.