Usually, If I really like the comic book or graphic novel, I don't like the movie quite as much, though I still enjoy it if it doesn't screw things up completely. Examples include: Lone Wolf and Cub, The Crow,Ghost World, V for Vendetta, and American Splendor. Nausicaa and the Valley of the Wind is really close, but I feel the comic book is a little bit better in the world-creation department and the ending was more poignant (of course, they are both by Miyazaki, so that's a special case). Mainstream superhero comics are hard to judge because there are so many different writers and inkers (I don't read them much because of that, though I can enjoy the symbolism and mythos in a movie).
There are a few movies that are better than the comic books, though:
Ghost in the Shell. It's a good graphic novel, but Shirow gets sidetracked by his love of shiny mecha and cheesy cheesecake (though he does both well). The movie really focuses on the theme without the clutter, and I can watch it again and again.
The Stormriders. Wing Shing Wa is the master of crazy kung-fu battles, but his story is all over the place and his characters lack personality. The movie distills what's great about the comic into a silly but super-fun cgi Hong Kong version of Clash of the Titans.
Lady Snowblood. This one is close, but Koike (adapting his own comic) cuts out some of the silliness and the totally shoehorned-in lesbian subtheme of the comic, leaving us nothing but sweet, sweet vengeance.
I haven't read the graphic novels of Hellboy,Diabolik, or History of Violence, or seen the movie of Persepolis. Any thoughts on these? Someday I'd like to compare the manga, anime and French live-action version of Rose of Versailles (AKA Lady Oscar), but I'm not sure I'm ready to lay out the considerable dough.
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