Let's be real, if you're asking about 18+ content, everyone's mind goes straight to Archive of Our Own, but I don't think that's the full story. AO3 does host an absolutely massive amount of explicit fic, and its tagging system is legendary for finding exactly what you're after, no matter how specific or niche. The filter for 'Explicit' is right there. However, a ton of the most popular, viral 18+ fics for certain fandoms still circulate on Tumblr via links or are posted directly on dedicated Discord servers. For some older fandoms, LiveJournal descendants like Dreamwidth still have incredible, classic stories you can't find elsewhere. So while AO3 is the central archive, the 'most popular' stories for a given ship might actually be spread out depending on where that fandom's community vibes.
Sometimes the absolute filthiest, most creative stuff lives in Google Docs links shared on Twitter or in private communities, precisely because it skirts platform rules. So popularity is weirdly decentralized for this category.