1. Death Note
As with many other things anime, DN wins first place. Sure, it was more or less faithful to the manga, but that is no excuse. As much as I love DN, I'd be the first to admit the second arc is dwarfed by the first one, but even so, the ending is an insult to the series. The events leading up to it and the final exposition is so unconvincing that I cannot shake the feeling the creators [of the manga] had run out of ideas and just came up with whatever convenient solution that seemed Death Notey. This one merely seems so, though. The logic is flawed, the setup is unbelievable, the punchline lacks force. In both the manga and the anime, Light dies like a dog, although in the anime, they at least allowed his to expire in quiet and away from the obnoxious people surrounding him in that warehouse. I also liked Mikami's reaction far better in the anime - choosing suicide rather than acting like a spoiled child. Like I said, insulting, both to the work and to its fans.
This one actually isn't all that bad, but it really provoked quite a reaction from me anyway. The anime apparently caught up with the manga, and they had to make up an ending. And what do they do with this freedom? Invalidate the entire purpose of the series. Well, of the main character. Clare's entire existence is due to her desire for vengeance upon the person who killed her caretaker (with apologies to bishou no Teresa), and when the opportunity presented itself - after a lot of pain, struggle and sacrifice - a certain annoying brat of a supporting character stops the execution. Bravo, idiots, bravo. This one pissed me off so much it turned into my very first post here.
If this list dealt with terrible anime with terrible endings, Spiral would win like a boss. A very confused boss. No focus at all. A boss who doesn't know what he's trying to say, so he keeps mumbling "blade children" for the entire time, only to leave the "mystery" unresolved. Yes. That's right. This series is like half an season. Actually, it's worse than that. The manga is rather long. The events portrayed here are nowhere near half of that occurs. Whoever the hell adapted this must be an idiot. What's worse, after I bothered to inform myself in detail about the happenings in the manga, I was rather glad I didn't bother to read it before informing myself - the grand resolution is so banal I still can't believe that was what they came up with. No focus, nothing to say, meander, meander, end vaguely. Horrible.
Yet another case of having to make up an ending due to incompleteness of the manga, GANTZ doesn't do too bad. The five or so final episodes go in a completely different direction than what the manga eventually became, but I'm okay with that, they weren't too bad. However, the ending itself is meaningless. A very feeble attempt at symbolism that tries to cover up for its lack of logic or meaning with vagueness and white. Lots of white. Eh. Talk about not trying.
5. Code Geass: Lelouch of the Rebellion
Although it didn't strike me as such because I watched the two seasons back to back, S1 has a really, really poor ending. It's basically a cliffhanger, with nothing, and I do mean nothing else. Not even a small attempt at trying to give any kind of closure to the viewer. Sure, it was obvious the sequel was a-coming, but it still seems like a nasty thing to do to your viewers and fans. However, they partially made up with it with S2 - aka one of the finest anime endings known to man (this one, at least).