Roald Dahl is one of the great ones. The BFG and The Witches were both among my favorite books when I was a kid. There was a dark side to his stories but only in the best way possible. It made his characters more interesting and "real"" despite his fantastical settings.
He was my FAVORITE author growing up. I read everything of his, even his autobiography about his childhood "Boy". I need to re-read some of his books... like The Twits. Somehow reading was so much more fun when I was a kid.