This is the first time I've used this feature (odd, since I programmed the damn site), but I'm tossing in my two cents on Sin City. I don't want to overwrite what Xrys has said, and I think just about every one of us is going to have something to say about this movie.
Its really hard for me to say a movie was the best ever. That's throwing it down next to the original Matrix, Pulp Fiction, the Ridick series, Blazing Saddles and The Gods Must Be Crazy. In fact, I'm not going to say any of those movies on that list are better then any the others. What I am going to say is that I have a short list of movies that are PERFECT. Movies I could never complain about, movies that are without flaw, movies that I would not wish to be different in ANY WAY. Sin City is one of those movies.
I have never read Sin City, but I have read other Frank Miller books. I have deep respect for The Dark Knight Returns and how it DEFINED Batman. I went into Sin City knowing Frank Miller, but not knowing this particular story, so my take on the movie is of Frank Miller's ability to create a movie. The result is that he is one of the finest filmmakers of our time.
The craft of the movie is breathtaking. Every moment feels the way it is supposed to feel. At times it is meant to feel like a movie, at times it is meant to feel like a comic book. There are times when Miller wants you to simply see an image and feel it with all your soul. This effect is something that until now, ONLY the medium of comics has mastered. Movies before now have tried the effect with moderate success, but Miller mastered this effect with film. You see an image on the screen in black and white... stark, heavy contrast. You feel it. You feel it with the eloquence of ink and paper. It gets you.
Color. The movie is black and white until you need to see a color. When you see that color, you love it. You respect it. Sin City is what Pleasantville wanted to be but never could.
Story. The characters are real. You believe them, you love them, they aren't good, they aren't evil, they are human. Many of them are super human, but in a very HUMAN way. They don't put on costumes and advertise their power. Most of them want to be left alone, live normal lives. They are super human, but not immortal. They can die and they know it. They know what they can do, what they can't. You feel their pain. Their powers are real.
Sin City is perfect. Dark, gritty, emotional, and human. I'm not even going to rate it, because it's not worthy to be rated by Jer of Splurd. Frank Miller, oh captain my captain: I salute you. You inspire everyone who has ever drawn Sequential Art. Thank you.