writers block
Fav- Red and Scott Pilgrim vs. the World. The first cuz it was expanded from short awesome Ellis comic to longer awesome Willis movie. The second cuz the movie IS the comic. And wrapped up all 6 comics beautifully.
Honorable Mentions: Hellboy (1 and Sword of Storms animated), Kick-Ass, Ghost World, Constantine/Hellblazer (the movie is far from the book but I like them both for their own merits), Iron Man, X-Men (1 and First Class), Sin City, The Spirit, V for Vendetta and Wanted (one of my top three re-watchable movies)
Least fav- Watchmen. It's my least fav because it's a great comic that the movie ruined by completely missing the entire message of the comic. As opposed to all the terrible comics turned into terrible movies (eg, Daredevil, Elektra, Howard the fucking Duck, etc.). I'd put Ghost Rider here too but I've never read the comic so I'll refrain from commenting.
Green Lantern, Batman (esp. the new ones), Superman (esp. the new one), The Hulk (Ed Norton version), The Crow (just the first one), Captain America, Blade (all three), Thor, Spiderman (first 2), Cowboys vs. Aliens, The Punisher (Thomas Jane version), Mystery Men, Men in Black (first one), League of Extraordinary Gentlemen... were all quite likable too. Did I forget anyone else worth mentioning? Hmm.