In my humble opinion, the worst movie I have ever seen was, and read why before you scream and yell and try to beat my head in with a stick, was Eragon. Not because it was genuinely terrible, it wasn't, and in fairness i haven't seen many really bad movies, but because I am a huge fan of the books and was gutted at what it was, essentially a collection of things from other movies without trying to be different, just hollywood showyness, totally emotionally dead, (especially painful from such an excellent emotional book) and leaving out what made the book great. For that I could never forgive it, so it gets my vote.
WHY!!! HOW COULD THEY DO THIS TO THE FANS!!!