Name An Anime That Had The Worst Ending For You.
Something that made you go 'what the hell was that' ... or something that made you want to break something or someone.
Akame ga kill had me like that, along with Elfen lied.
PEACE.

99% of romance anime.
How are they supposed to end? =S
First they should actually have an ending. Most of romances have problem with that.
That I would agree with. Unresolved romances are a bitch to watch. I wouldn't say the make up '99%' of romance shows though.
I'm pretty sure that out of 111 days worth of anime that I have on my list probably 80-85%(or even more) of them are romance anime and looking on my "Resolved romance" list which has about 50 anime on it, and now out of these 50, let's find something with a decent one. Yeah, it's not 99% but I'm positive that about 85-90% of romance anime I've watched didn't end at all.
mahou sensou.
the whole show was rather trash, but the ending; i can't even.
@Insti will back me up on this.
Darker than Black. The initial ending is a relatively confusing reveal and doesn't really wrap anything up or bring closure. And even though they never made a second season, if they had it could probably be considered one long drawn-out terrible ending that concludes with them killing the best character in the show for no good reason.
Hyouka's I think was pretty bad because it decided to focus on the relationship I gave zero shits about, and even then, it doesn't actually go for any resolution. It's basically a tease of a pairing I didn't really care about, instead of the pairing that was more interesting.
Angel Beats I have mixed feelings about because on one hand I do think the whole graduation was pretty emotional, but by God I could not give a shit about Kaname and the MC's crush on her felt tackled on, so his crying after she leaves just looks silly.
Ping Pong The Animation because fuck Peco.
Noragami's was a half-assed anime-original effort that borrows strengths from other arcs and makes them worse.
Chaika's 2nd Season ending was pretty bad because it was entirely rushed (even moreso than the rest of the series, which is already pretty rushed), and makes some character's actions look really arbitrary.
Paranoia Agent's is a big pile of garbage that takes all the Satoshi Kon elements that suck and sticks them together to make something with no impact whatsoever.
But we all know the real winner for worst anime ending is Fate/Zero