Archive of Our Own, hands down, but that’s if you’re willing to wade through a mountain of content. Honestly, sometimes the sheer volume feels overwhelming. I love AO3’s tagging system and how you can filter for pretty much anything, but discovery for the truly small, weird fandoms can still be hit or miss. You need to know what you’re looking for, or else you’re just scrolling forever.
For the real obscure stuff, sometimes I’ve had better luck on Dreamwidth communities or even Tumblr blogs dedicated to a single ship. The content is more curated by passionate people, even if there’s less of it overall. FF.net is still a giant, but its search and tagging are so clunky for niche things; you’re basically relying on luck and decade-old summaries.