[Kotohono]

Thinking about I'd have to say undoubtedly Yumekui Merry

Spoiler:

I've never seen such built up for a strong villain end in such stupid and confusing nonsensical for them to be defeated, and one of the characters built up to fight them only to make their plan be utterly useless, and instead depended on about 5 different deus ex machina pulled between various characters to achieve their victory and ignore rules previously put in place by the series with each of them making less sense than the last. The whole thing ultimate not only made no sense, but somehow managed to make such a battle extremely boring.

And to make matters worse, the characters either just acted like the real final villain is gone and everything was somehow solved because they defeated this villain, and are giving happily ever after type ending when it made no freaking sense, why foreshadow a final villain to only pretend he doesn't exist later, I DO NOT UNDERSTAND THIS LOGIC AT ALL.