Clannad is... Many things. It started off as a PC visual novel in 2004, made its way to anime in 2007, and has been adapted into no fewer than four mangas. For one reason or another someone out there just can't get enough Clannad. Maybe they're just captivated by the incredibly creepy character faces.
Whatever the reason, Clannad cannot -- will not -- be stopped. Which gives us plenty of time to consider its totally bizarre story structure. Particularly the ending, true to the spirit of this list, comes completely out of nowhere.
Like School Days the story begins as a romantic slice of life anime. Here, however, the characters aren't a bunch of scumbags so much as... Exactly what you'd expect from a romantic slice of life anime. For the longest time the greatest hardship the principle characters -- Tomoya and Nagisa -- endure is whether or not to hold hands.
That escalates in the latter half of the second season. It escalates hard. By this point Tomoya and Nagisa have wound up married, carrying out their boring-ass lives as a couple. They make the perfect average decision to have a probably-normal child together. In most cases this would be the end. But no. Not for Clannad.
Nagisa succumbs to a long-time illness during childbirth, and dies just seconds after her daughter is born. That leaves Tomoya alone with their kid for five years -- a time which is not entirely kind to his psyche. He takes up drinking and smoking as nighttime hobbies instead of, you know, raising her. Though even this quickly fades as Tomoya learns to love his daughter despite his circumstances.
So of course she dies too.
Tomoya -- already perhaps not the best a dealing with grief -- enters a... dream world? A parallel dimension? Maybe The Matrix. It doesn't really matter, because it's all an excuse for the story to rewrite itself. Tomoya travels back in time, ignoring genre convention and the complete lack of anything supernatural up to this point, to the moment Nagisa died. Except this time she doesn't for reasons that are never given. Here the two get the happy ending everyone expected, and the viewer gets to question why they bothered watching the last six episodes.
Here we have another mecha anime produced by the same company as Neon Genesis Evangelion. Before you feel like we're repeating ourselves, however, know that Gurren Lagann is the perfect example of an ending that's both completely bananas and weak-in-the-knees satisfying.
Even before the ending the series is full of absurd twists and turns. What starts as a post-apocalyptic tale of man versus monster versus giant robot evolves into something much, much more. Halfway through the series the viewer is transported several years into the future. Team Dai-Gurren, the protagonists, have defeated the wasteland king Lordgenome, and are rebuilding society.
Suddenly, we discover this was maybe not the best idea in the short-term. It turns out Lordgenome was once a hero of the human species, having turned back a much greater threat in his own time. These "Anti-Spiral" aliens just couldn't abide by humanity's capacity for endless growth, and sought to wipe them out. Lordgenome wiped out most, but not all, of his own kind in a bid to keep the Anti-Spirals from driving them extinct.
In the series' second arc the E.T.s return to wreck shop. It turns out that Nia, girlfriend of series protagonist Simon, was a sort of alien P.A. system this whole time. So when the Anti-Spirals notice human civilization rising again Nia lets everyone know they're fucked, calls in the cavalry, and books it into space just as a new purge begins.
From there this once earthbound, post-apocalyptic anime makes the logical leap into space. Team Dai-Gurren retrofits an colony ship for combat and makes its way to Nia and the rest of the Anti-Spirals. What follows is a final fight on a universal scale. Literally. A mix of Anti-Spiral technology and Spiral (i.e. human) willpower makes the confrontation so big that those left behind can see the battle from Earth. Including the Anti-Spirals chucking actual galaxies like fastballs.
After a load of character deaths Simon makes it to the finish line only to end things with a fistfight against the lead Anti-Spiral. Simon, the survivors, and a Nia now no longer under alien control return to Earth. Everyone still alive gets a happy ending, punctuated by Nia and Simon's wedding... At which point Nia dissolves into space dust since her power source, the Anti-Spirals, are gone.
The quintessential happy ending.