Fight Club, forever on the top of my list.
Donnie Darko because who doesn't love a good mindfuck?
Requiem For A Dream because Darren Aronofsky is a genius.
And let's add Pi to that, too.
Inception, the movie that got me into the mindfuck genre.
Eternal Sunshine Of The Spotless Mind for the quirky romantic in me.


And lastly, a movie I'd recommend everyone in this forum to watch—Waking Life. I think you can tell from the title that it's got to do with something

Seriously, though. At least check out the trailer. I would post a link, but I'm not allowed to because I'm new to the forum. Anyway, Google it!
I swear it'll be worth it.