Re:Zero Watching Him Die Again and Again by Reactionist
It's one of the most successful "Anime cast Watch their own show" fanfic to exists.
Especially with Canon MC ability to Return By Death, it pretty much a very interesting to watch how they would react to their counterpart in alternative timeline.
it has around 400,000 words within 14 chapter. So It's has more details and have more words than average Scribble Hub or even Royal Road stories.
Its a good entry story i guess, although a little bit dry compared to Decent stories.
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But if the first fanfic manga I've ever read, it's Doraemon "Doujin". It's not the 18+ one, a manga fanfic based of original story is also called "doujin" in Japan, even doesn't matter if it contain 18+ material. If it contains 18+ material, it's called "Hentai".
The manga It tells the story of show how Doraemon story would end after Nobita himself determined to repair the blue cat robot, becoming the most prominent researcher of their time.
Took me years to find out that its not the real ending, as the original Doraemon author didn't really make official ending manga and only put vague bits of how's Nobita future is gonna marrying his childhood crush, "Shizuka", while not directly showing what happened to Doraemon.