Oh, yeah. For the record books:
The Coen Brothers: Fargo was great, but it's a flavor-of-the-week film. I'm going with The Man Who Wasn't There. I'm such a sucker for Scarlett Johansen.
Sofia Coppola: Lost in Translation. Hands down. If she ever tops this, I'd be surprised.
Spike Lee: Do the Right Thing. I'm an unnatural fan of Summer of Sam, but it doesn't come close to the poignancy of Do the Right Thing.
Quentin Tarantino: Pulp Fiction, but he's got the potential to do better. He keeps getting stuck in these genre films and he's losing his credibility as as "original" director.
Alfred Hitchcock: Rear Window. Suspenseful, inspired a recent remake (or "modernization").
Michel Gondry: Eternal Sunshine of the Spotless Mind. A beautiful film that realizes the beauty of tragic love and also the beauty of raw, un-CGI'd cinema.
Stanley Kubrick: A Clockwork Orange. Actually, call it a toss-up between that and Dr. Strangelove, or How I Learned to Stop Worrying and Love the Bomb. They're both so hard on society, but they both say it very well. One warps you and leaves you disturbed, the other makes you laugh and wonder why you're laughing.
Ridley Scott: Blade Runner (Director's Cut, not original). I've always interpreted this movie as a commentary on the nature of our justice system, which seems to be a common theme in other movies based on/novels written by Philip K. Dick.
Christopher Nolan: Memento. An original use of the disjointed time editing style.
Danny Boyle: 28 Days Later. Like Trainspotting and all, but 28 Days Later actually redefined what it meant to make a horror movie. Since the release of this film, we have seen a dramatic increase in the quality of horror films. See The Descent for proof (but that's another British film, so does that disqualify my suggestion?).
P.T. Anderson: Jesus, how does one decide on a single film of his? They're all masterpieces!