The worst one to me was Watchers by Dean Koontz. I love the book. I think it's his best work. So imagine my joy when while reading the book I found there was a movie based on the book. I ordered it through Amazon.
Imagine my frustration when I saw what had been done to the story! Every detail was changed except the most basic element. The MC had been changed from an ex marine to a high school kid! The love of his life in the book had become his mother!
Oh gawd. There aren't enough words to describe how wrong that is. (shudder) And sort of funny in a horrid way.
Me? As a reader, I can't stand when filmmakers change novels. I understand the need to cut things out, tweak things here and there. Movies are a different medium and they usually only have two hours.
But there's a big difference between cutting things, deciding you'll focus on
this aspect of the novel and leave out such and such subplots, and actually
changing the heart of a novel. When females become males, adult men become children, when sad endings become happy or vice versa... I wonder why the hell they didn't just use an original screenplay.
So, yeah. I'd also be cursing to the bank.
added: Of course, there are exceptions. If the film was a beautiful piece of art in itself (not just a moneymaker) then I'd probably be forgiving.