Sure, arguments can be made for Superman [the first with Christopher Reeve and directed by Richard Donner], the original Crow movie [with Brandon Lee] and even this year's Batman Begins. But for my money, Sin City is the cream of the crop as far as getting to the screen what was on the comic page.
That may change next year when Warner Brother's releases 300. Interestingly enough, 300, like Sin City is based on a graphic novel by Frank Miller. What's also interesting is that Zack Snyder, 300's director, will make extensive use of the same "green screen" techniques employed by Robert Rodriguez and Frank Miller when they brought Sin City to life for the big screen.
Check out the official 300 web site and you'll see that Snyder is a huge fan of Frank Miller's work. I believe that Snyder will bring the same sense of respect and ability to 300 that he brought to Dawn of the Dead. And if that's the case, we'll not only get a great movie experience but also possibly the best comics to movie adaptation ever.