The problem is that, and while we have archived almost all the text files from the booru as of August 2020, downloading a text file from the booru does not preserve its tags, nor its title. I am indebted to @meganonymous and a few other people for their work on finding text files, but its not an easy task.
The biggest loss to my understanding is Abandoned Angel. It was deleted from the booru and only a portion of it, as well as one fanart, remains.
You have to specify whether you mean text, or comics. There are some comics that are pretty long.
If text, I really liked the work of Oracle. His pastebin can be seen here, and some of Marcusmaximus’s hugbox pictures are based off these: Oracle's Pastebin - Pastebin.com
I really enjoyed the four fics @Carpdime wrote of Avocado way back in the day, I reuploaded two of them so far, one of them being The New Mummah. Some of Carp’s recent work in what I call the “Avocadoverse” is heavy in depicting slice-of-life and everyday actions in a world where fluffies are commonplace and “accepted” as pets, and this applies to both his art and his fiction.
Phantomfluffy did Fluffy neighbours, which was a pretty nice text series, and he also did art of it: https://fluffy-community.com/t/phantomfluffys-bunny-her-foals-and-also-citrus-artist-phantomfluffy-fluffybooru-id-3918-3800-11074-4466/7623 Sadly, he never really continued it.
Squeakyfriend wrote some really nice wierdbox of Doctor Crazystein.
One of my favourite stories is one by Wangiew_Wick, which was a tribute he did to Quickhorn. Even though the picture itself is on DNP, the tribute story is not, and I consider that story he did to be his best work.
I’m sure there’s more, but part of the problem with reading fiction, unlike with comics and art, is that its more time-intensive.
As for comics and art, well, there’s a reason why I’m working on my #sam_adams_guide lmao! But offhand, my favourite continuous picture stories have included Plum, Postfluff, NISC, the Rejects, Rosie & Blue, Smokey, and of course, Avocado.