I keep a dedicated bookmark folder for Bungou Stray Dogs fic, and the Kunikida/Dazai corner is predictably sprawling. Ao3's tagging system is non-negotiable for me—being able to filter by 'Established Relationship', 'Angst with a Happy Ending', or 'Canon-Typical Violence' lets me find exactly the mood I'm after. It's less about 'best' and more about curation. A specific author I follow, 'KyoukaIzu', writes them with this incredible friction, all idealism versus nihilism without either side being a caricature. You'll find high-concept AUs on Tumblr sometimes, but keeping track of multi-chapter works there is a mess.
Honestly, my second stop is often older, forum-style archives like Fanfiction.net. The quality is wildly inconsistent, but there's a raw, early-fandom energy in some of those stories from 2016-2018 that I kinda miss. You have to dig through a lot of OOC fluff, but stumbling on a gem that nails Kunikida's repressed frustration feels like a reward.
Archive of Our Own is definitely the main hub now, but I wouldn't sleep on Dreamwidth communities either. They tend to host tighter-knit, often more experimental or meta-focused fic exchanges, which can produce some really sharp character studies you don't always see on the bigger platforms.