Se7en is my pick also, even though I kinda hated that it's not just called 'Seven'.

Great performance from Spacey and it's an interesting concept for a film and for a killer. I thought some of the other characters were a little flat, which prevents me from thinking of it as an outright classic, but it was highly entertaining and stands the test of time.
I honestly never really responded that strongly to Fight Club. All of the hype I had heard before seeing the movie might have turned me off it somehow. I still think it's pretty good, but don't think it was necessarily worthy of the huge amount of praise that was heaped upon it.
The Social Network was a decent enough film, with a cool soundtrack and some impressive performances (particularly from the underrated Andrew Garfield), but the story didn't go anywhere at all and the ending was contrived, patronising and stupid.
Haven't seen any of the others, except for Alien 3 as a kid. Doubt that would be ranking #1 if I watched it again.
