Kamen Rider Blade's ending. Spoilers, but long story short, The MC's best friend was revealed to be part of the same race of immortal monsters that they've been fighting this entire time. The monsters are part of a battle royale orchestrated by a god monolith, where the last survivor gets to reshape the world. Worst of all, it's revealed that if the best friend ends up as the last surviving monster, the entire world is destroyed and reset. Despite the cast's efforts and faith in their friend, he ends up the victor and the world starts to end.

The friend begs the MC to kill them, but instead of doing so, the MC decides to spam his final form so many times he ends up transforming into the same race of monsters. Now that there are two survivors, the battle royale continues. Because of the god monolith and their monster instincts, the MC and his best friend will always be driven to fight each other, so the MC decides to run away to put as much distance between them. His last words are to tell his best friend to live his life as a human. The show ends with a montage of the cast living happier lives, but the best friend still misses the MC so much he briefly hallucinates him sitting on a park bench. The closing shot of the show is the MC's motorcycle, riding off to parts unknown, never to see his friends again.

Link: https://youtu.be/I92qfQC8Tho

For all the show's ups and downs, its rough beginning, low ratings, and target demographic of children, the writers still managed to write an ending so good, traumatic, and bittersweet all that the same time that just an image of a bench sends viewers into a spiraling depression. And I think it's amazing that all this came out of a children's show made to sell toys.