Dolores Umbridge
Umbridge...the absolute worse teacher they ever hide. She was forced upon Dumbledore by Fudge who wanted to have a ministry worker at Hogwarts to keep an eye on how things were run. She was intent on teaching the children theory of defense but wouldn't discuss it (yeah that's a great way to teach.) She was made High Inquisitor and slowly did a Hitler and made herself even more powerful than Dumbledore by issuing new rules. Her detentions included a quill that used your own blood to write on the paper by scraping the skin away from the back of your hand with the words you had just written. In the end she was led into the forbidden forest and through Hermione's genius, was trampled on by Centaurs that in the end made her crazy.