Pulp Fiction is def first. then, after that... it's a bit of a toss up. they're such different films.
It also pains me to leave out a Fincher film, though it just depends on the day of the week, really. (PLus, Fincher claims All the Presidents Men was a huge influence on him - and I really love that style of filmmaking) Sometimes, I'm in a Zodiac mood, others a Social Network or Seven groove. and then there's Fight Club, which I've kind of outgrown, but still remains one of the funniest movies I've ever seen.
edit: ^fuck yeah Pulp Fiction. and David Fincher. 90s, fuck yeah
Eternal Sunshine of the Spotless Mind. Originally I saw this movie with a friend just randomly in theatre when it came out. Really had no idea what the movie was about, but thought the cast had promise. Had to see it another three times to wrap my head around it all. Great performances from everyone, I love the way they merge the sequences and shoot them. I enjoy a lot of movies, but this is far and away my favorite. If you've never seen it, I couldn't recommend it enough.