Ooo, fun!
Oldboy - I never get tired of watching this film. Contains one of the greatest fight scenes EVER made, wherein they manage to make one dude with a hammer taking on about 20 other dudes and winning look realistic. And it was shot all in one take. Park Chan-Wook for the WIN. (Here's the scene, if you're interested,
http://www.youtube.com/watch?v=Wha0brbb_44 )
And some more, in no particular order:
Halloween (The original. Go away, Rob Zombie.)
Clerks & Clerks 2
The Hitcher (The original. Go away, Sean Bean. I want to like you, but you are not Rutger Hauer. Stick to Sharpe's Rifles.)
The Shining (Kubrick's)
Silence of the Lambs
Escape from New York
Carpenter's The Thing
Audition
Martyrs
Dawn of the Dead (The original. Go away, Zack Snyder.)
Night of the Living Dead
Teeth
Feast
Midnight Express
The Departed
Reservoir Dogs
Labyrinth
The Princess Bride
Shaun of the Dead
Hot Fuzz
The Evil Dead series
Lost Boys
Tale of Two Sisters
Original Star Wars trilogy
Robert Rodriguez's Mexico trilogy
Hard Candy
Faster, Pussycat! Kill! Kill!
Three Extremes
The Descent (with the original UK ending, not the American theatrical release ending)
Guess I should stop now. Many of those are horror films, because that's kind of my thing.