People who refuse to turn their high beams off when there's oncoming traffic, even when you flash your high beams back at them repeatedly. Also people who leave their high beams when they're right behind you. That's even worse, especially since you can't flash back at them and there's nothing you can really do about it.
Also people who don't turn their headlights on at all in rain/fog/snow, or at dawn/dusk, or sometimes even when it's totally dark out!