All my teachers this year rock (Especially my American Citizenship and English 9 Honors), but the single worst one I had was probably my Kindergarten teacher. I didn't know it at the time, but my 5th grade year my classroom was near hers and I just noticed how she was always screaming at her kids. And I remember distinctly her telling me once "Why can't you draw like him [some artistic kid], Garth?". Bitch.
But since my life in my early years was basically one long LSD trip without the LSD, I'm not sure if she was always that bad.
My French teacher in 7th and 8th grade sucked, but she wasn't so much mean as just very flaky and didn't help you at all. She was about 418 years old, so we came up with hilarious stories about how she taught God French, and how her alter ego was a lady named "Martha Washington".
EDIT: I just realized my all-time worst teacher. My middle school art teacher. Her boobs were about down to her knees, and she once took my paper for the next class because I was writing my name on it. I ended up getting a 0 for it. Another time I used incorrect complimetary colors in a piece of op art (She gave us a choice of three, I used blue and yellow instead of purple and yellow), so I got a 0 on that. Damn, she was a bitch.
My worst teacher this year is probably my Algebra teacher, not because he's mean or even bad - he's actually a really awesome guy - but the class sucks, it's hot in his room, and he screams a lot to keep us awake. Like a minister. But again, the teacher himself is really awesome. And when I say he screams to keep us awake, he SCREAMS. This is some sample dialouge.
"What happens when you divide by zero?"
[the people awake stay silent]
"No one? When you divide by zero, it goes BOOM!!!!!!"
Then we're awake for the rest of the class.