In my opinion, the worst teacher is the one that doesn't want to be there, but says nothing - and does nothing.
They slip in late, long lunch, leave early. Never involved in activities unless under duress. Avoid any real work like the plague. Their clothes are as tired and sad as they are. Their exams and courses are a disaster. Don't invest a satang in their wardrobe or to better themselves. Repeatedly receive borderline evaluations.
The school just hopes they quit each year. They have no clue how to behave around professional adults. Their work if done at all is late. These are the negative teachers that wear me down.