Aunt Maria (required reading)

The witch's of aunt Maria come aross as conventional old ladies at first but you quickly learn that they are in complete control of the town that they live in and are using their witchcraft to have it functional exactly the way they want it too. 

In many ways, the idea of the witches being in control is very conventional to the archetype of the classic witch. Their subtle manipulation are staples of stories involving witches.

It doesn't take long for the readers to discover how nasty the witches are, particularly aunt Maria herself. The novel uses this slow reveal most effectively. Taking Aunt maria from unlikable old lady to a force of evil. This is really what makes the character of the witch great and I think Diana Wynne Jones understands that and uses it effectively.