Why I have a very bad view of Gilgamesh
Gilgamesh (also ANN, MAL) is a 2003 anime series with a distinct and somewhat offputting art style and a bunch of reasonably interesting characters and things going on. That's about all you can read of this entry if you want to avoid spoilers, but this is one time I think that you should embrace them even if you're planning to watch the show and read on.
(If you don't know Gilgamesh and want to follow the rest of this entry I recommend reading the summaries above. Note that the Wikipedia page has full spoilers for the ending.)
I'm generally not someone who is deeply sensitive about endings. It's not that I'm indifferent to their quality, but by and large my feeling is that even a bad, disappointing, or incoherent ending merely makes the overall show unsatisfying; the good work that the show did before its ending remains even for endings that you can't just basically ignore outright. I also have no particular problems with non-ending endings (I've long since become inured to incomplete works in all mediums), although of course I prefer ones that feel complete. As a side note I find this a useful attitude to cultivate because many shows have problems with their endings.
(My standards for what makes an ending feel complete is probably different from most people's, but that's another entry.)
Gilgamesh is a very, very big exception. It has the worst ending I can remember watching, an ending that is so bad that it retroactively tainted and ruined the rest of the show for me. Even today it makes me angry (never mind what it did at the time). This is not because the ending is technically bad; it is perfectly coherent, well animated, relatively easy to follow, and so on. The problem is what the ending of the show actually is and there are two parts to that.
Throughout the run of Gilgamesh the show built up the question of why things in the Heaven's Gate facility happened as a big mystery (which always risked ordinary disappointment). The lesser problem with the ending is that the show gives this mystery the most asinine and petty explanation it could, and does so out of the blue. It turns out that all of the massively big events underlying the show happened because of (drum roll please) love and jealousy. For extra points, the character involved had no idea of this and the show doesn't give us any hints until this is sprung on us at the last moment. It is a giant 'was that it?' moment and probably the most unsatisfying explanation for a big mystery that I've ever seen a show pull.
The greater problem with the ending is that the show goes on to give us a 'rocks fall, everyone dies' ending in which the world is destroyed and the protagonists along with it. All of the protagonists' heroism to date in working against that is useless; they lose. In fact they are quite literally slaughtered, one by one, often in cruel and deliberately humiliating ways. The show is clearly not happy to just kill them, it wants them to suffer on the way down. This sort of death is an excellent way to make me hate a show.
(It is not quite carnography because the show doesn't lovingly dwell on the blood and carnage of their deaths; in fact I remember it as relatively antiseptic as far as that goes. It just wants to grind into us that the characters are totally ineffective at resisting their deaths even when, where, and how they had previously been shown as competent, and sometimes to humiliate the characters in the process.)
Oh, and all of this comes out of the blue. There are no real hints in the show about either part of the ending, nothing to prepare us for the joke of the ultimate cause and the total bleakness of the ending. My memory is that up until basically the last episode it looks like we're on track for a good ending.
This is cruel nihilism on an epic scale. I've never seen a show extend such a giant middle finger to everyone watching as Gilgamesh did and as a result the ending utterly ruins the show for me. It's impossible to think back to what happened before with any good feelings, partly because the protagonists spend all show fighting against this ending and I know that they are just going to their deaths.
(The middle finger is much worse than End of Evangelion and comes much more out of nowhere.)