I gotta say, lots of good taste in this here thread...
As for me, I say The Thin Man, the 1934 original. You know how every crime comedy with a male and female lead rips off Moonlighting? Watch this to see where Moonlighting got its tricks. (And if you like it the sequels generally aren't bad.)
Also Brotherhood of the Wolf (Le Pacte des Loups), a French costumed period martial arts romantic fantasy action horror mini epic. It is utterly bonkers, and sublime. Watch it just for the best martial arts sequence in the rain I've ever seen. And a naked Monica Bellucci in her prime (enabling arguably the best dissolve in the history of cinema).
Dark City, if you like German Expressionism, or stylized noir in general. It is really good. It is also IMPERATIVE THAT YOU WATCH THE DIRECTOR'S CUT, NOT WATCH ANY BIT OF THE ORIGINAL CUT, TALK TO NO ONE ABOUT IT AND NOT CHECK IT OUT ON YOUTUBE OR READ THE REVIEWS. The original cut reveals WAY too much too soon, and everything and everyone will have assumed you know what's up and spoil it. It's 66 on Metacritic and 75 on Rotten Tomatoes if you care about that sort of thing, and it's one of Roger Ebert's Greatest Films of All Time. I'd link to a trailer, but it's one of those infuriating trailers that's brilliantly executed but gives away half the movie. Watch it after the movie because it's a really nice trailer.
The Fall, Tarsem Singh's last film. Opinions differ on the quality of the story (I think it's great), but it has indisputably some of the best imagery ever put on celluloid. No CGI (or visual effects, really) in the entire film. Knowing that, absolutely watch the trailer for this one.
Three Kings, the '99 action movie. Really fun and really smart. Also interesting visuals.