I'm quantifying Best and Worst in this as most closely/least closely adapted the source material.
Best Novel to Movie: Misery
Best Comic to Movie: Going by my criteria above, I'm going to say Superman 1/2, with an honerable mention to Iron Man
Best Play to Movie: Kennith Branagh's Hamlet (as much as I hate that play, he did it well). Another honorable mention to Noises Off.
Best Musical to Movie: Chicago
Worst Novel to Movie: A Stitch in Time
Worst Comic to Movie: Catwoman, dishonorable mention to the Reb Brown version of Captain America
Worst Play to Movie: ANY of the shitty Miramax "re-imaginings" of Shakespeare, such as O or 10 things I hate about you.
Worst Musical to Movie: Phantom of the Opera (hear Gerard Butler sing terribly!)