I voted for nightmare then regretted it two seconds later, but this poll doesn't allow me to revote. For me, it's a really close call between Nightmare and Fight Club - but while Nightmare was my favorite movie for several years, and Fight Club was never my absolute favorite in the same way - I think at this point in my life I prefer Fight Club.
Back to the Future - again, a movie that made a big impact on me when I first saw it, and I voted on it for the preliminaries, but not against this competition. Memento, for me, is a non-starter - while it is certainly good, it never really worked for me as well as it did for most people.