A piece of media you think is truly one of a kind.
It can be anything—manga/manhua/manhwa, anime, games, movies, or anything you feel has nothing like it.
For me, it’s Amnesia: The Dark Descent. It’s the only game that really got horror or "scary" right. Instead of relying on jump scares, it focuses on atmosphere and the slow build-up to encountering the monster. Even the developers took years and several sequels before creating something on that level again with The Bunker.