Without a doubt, 100%, with every ounce of my heart and mind, Donnie Darko.
I really can't do it any justice, but the acting was imensley believable, the characters enticing, and the whole story a beautiful mix of love and meloncholy. I'm not as partial to Richard Kelly's later work, but this movie is simply brilliant.
And really, what other movies have a main character who wears a demonic bunny suit?
Also, now that I've seen more strange films, I can add to my list...
2. A Clockwork Orange - There's a reason this movie is referenced so often. It deserves it. To see that now it's 40 years old and everything is still as sharp as when Kubrick first put it out, I can't help but love every second of it.
3. Being John Malkovich - quite the concept we have here. Very interesting, I always like to question if the ending was just or not.
4. The Machinist - Gotta love the way they shot it; I can't pick out more than a handful of shots with sunlight present. Add an emaciated Christian Bale and a story consisting of a mix of memory, paranoia, and an uncertain reality, and you've got a great film.
5. Requiem for a Dream - This one is my least favorite, as for some reason Jared Leto annoys me in his main role. I can't explain it, it just makes the film a bit more difficult for me. Still, it's an interesting concept; exploring addiction made for an interesting movie.
(Can we all pretend S. Darko was never made?)