I think the worst anime I got more than halfway through was Manabi Straight.
I got through it because I was somewhat compelled by the earlier episodes, and the girls were somewhat cute.
But man. I raged when they started fagging over that school fair. That blue haired cat chick pissed me off frequently, as did the Sakaki clone. To this day, I still like the other three main characters, to some degree. But too much pissed me off for me to like anything else about it.