I'm not sure,, I'm not much of a movie person, but I think some of my top picks would be
Primer - I didn't finish reading the description before deciding to definitely watch the movie. I first encountered it when I red somewhere that "anybody who claims he fully understands what's going on in Primer after seeing it just once is either a savant or a liar." which intrigued me, then I'll try to read something along the lines of - "The director was a Maths student who took a course in physics in order to create a film about time travel." I stopped reading at that point. It was enough -I love time-travel and plots that puzzle you, so I watched Primer. After I did it I came back to that description and the very next sentence was that the movie was made with about $7000 budget. I can honestly tell you that if the first sentence hadn't convinced me, the second one would have. It shows dedication to do a low budget movie and I am a fan of that.
Basic - two investigators try to solve a case in the military and hit intrigue and conspiracy. Things seem stranger with each new "fact" they turn up.
The Mask - for some reason, I find this movie really amusing. I love it and I've watched it more times than any other (including the mandatory Christmas movies who are sometimes shown again once or twice during the year). Heck, one time I watched the movie several times in a row for a few hours and it was still good. It will have a special place in my heart forever.
The Cube trilogy - I like each one of the movies, including the second and third one. I just wander why haven't they picked up the franchise and continued it, then again I really don't want them to turn it into a enormous string of sequels that lose their meaning pretty soon (you know, like Friday the 13th, Nightmare on Elm Street, Saw).
Fight Club - just classic.