In no particular order, and in arbitrary number:
> Oldboy - the original Korean one. Amazingly done, beautifully acted, with a hell of a twist.
> Apocalypse Now - no other movie really shows the mass insanity of war. (The Wrath of Aguirre is thematically similar, and also amazing. Klaus Kinski was batshit insane.)
> Se7en - pretty much the perfect serial killer mystery.
> The Crow - not necessarily one of the "best" movies I've ever seen, but it still just grabs me: the soundtrack is incredible, and the whole thing centers around a love story. Great cast, surprisingly funny in parts, and the whole thing is of course haunted by the death of Brandon Lee. My wife and I have a tradition of watching it every year on October 30.
> Dr. Strangelove - the single most genius bit of satire I've ever seen. I would also put Robocop under the umbrella of satire, and it too is fantastic.
> Heat - the perfect heist. (The Town is also really good, and definitely was inspired by Heat).
> The Cell - the story is whatever, but worth it for the visuals alone.
> Pi - a genius mathematician is haunted by what he's trying to do. Darren Aronofsky's first movie.
> The Blues Brothers - my favorite musical of all time. Amazing music, plus car chases and it doesn't forget to be funny.
> Super Troopers - smart enough not to annoy me, but still fun and silly.