"American Splendor" (Shari Springer Berman, Robert Pulcini, 2003) There's a lot of debate among graphic connoisseurs (comic-book fans) about which of the glut of "spandex movies" have captured the "essence" of its three-color source.
It's a puzzler, because the media communicate differently. Comics create a story-telling flow through still images--the reader must fill in the gaps* and that covers a lot of story-telling problems and physical impossibilities that films, if they are to represent reality, have to compromise. The geeks argue the "Spider-man" films are a good representative of the comic, but it takes more than re-creating iconic poses. There are those in my circle who consider the first "Hellboy" film to closely follow its graphic predecessor, and it does to a degree, though the insertion of an unlikely "Beauty and the Beast" romance is more in line with Hollywood sensibilities than Mike Mignola's. For a more literal translation of a comic source Robert Rodriguez took Frank Miller's "Sin City" black and white minimalist post-card approach and used it as a storyboard for his film--which exposed the heavy-breathing hyper-silliness of the source material.
The movie I'd vote for? Well, obviously "American Splendor" --the story of schlubby guy Harvey Pekar who chronicles his sorry-ass life in a comic book that he writes and is drawn by various illustrators, achieving something like a mosaic effect, much as the movie does. It (the movie) is, mostly dramatized--Paul Giamatti plays Pekar, and it was his break-out role--but sometimes illustrated, animated, and the real Harvey even pops on to give a dose of reality. In form and content, the film becomes the comic while not straying away from movie "fundamentals" of pace and movement, while also incorporating graphic techniques to give a sense of the book's "style." Of every "comic" movie I've seen, "American Splendor" gives you a sense of the worth of its source, while not compromising it. It may even, like any good film adaptation, inspire you to seek out the source material.