[Magnugiga]

Decade and post-switch Hibiki were frustrating experience, but at least with those I already knew beforehand and had lowered my expectation accordingly (though I still ended up being disappointed somehow )

The one that I hadn't expected was Build, I suppose. Really liked the beginning, and the ending's not bad, but almost the entire middle was ruined by increasingly stale drama/comedy (ymmv) and one of the most tiresome villains I've ever seen.

If it counts: Garo Crimson Moon (the second anime series). I was hyped for its setting and because of the really enjoyable previous season... this one seriously sucked, though. Terrible plot, character development, and production value all around.