Probably for me has to be my most hated anime, Popotan. A group of four sisters (I think they are sisters) give up normal life to go on a quest as far as I'm aware they are not told the goal of, by means of a huge time-travelling mansion. And their reason form all this? Because the older green haired sister can hear daffodil flowers (called Popotan) talk to her when she puts her ears close, and they basically tell her what to do (which isn't really anything at all) during these different time periods. None of the characters really understand anything aside from that it's their "destiny" or some shit. For reasons either I forget, or aren't explained, a man who is a spiritual entity tries to stop them, and I think the flowers end up forming a semi-nude female spirit as well.
Nonsensical plot aside, you feel like you're watching a little kids show, until the heavy needless use of nudity rear's it's head. Oh and one of the sisters who pretty much encourages all the nudity and is topless herself is about 5 years old.
Second place goes to Oretachi ni Tsubasa wa Nai: Under the Innocent Sky, also know in the English version as We Without Wings (somehow there is a currently airing dub of this crap). Reading discussions on this anime, i think it was unanimous that nothing makes any sense until midway, and then a bait and switch twist in the very final episodes (which asks more questions than it answers). No explaining, aside from the fact that it's still just an ecchi harem, can really do it justice, you'll have to watch it for yourself.
