Catherine Breillat. Worst - filmmaker - ever.
That's my opinion, I'm sticking to it. She has yet to make a film that didn't make me cringe with her obviously misanthropic streaks.
I expect a lot of people in this thread will mention any film by Uwe Boll. And they'd be right to.
The Michael Bay-produced Texas Chainsaw Massacre name-only remake - that was painful.
This thread is not about entertainingly bad films. Films by Lloyd Kaufman or Ed Wood are exempt in my mind here. This thread is for the painfully bad films.