Honestly, it's less about a single "best" spot and more about which corners of the fandom you vibe with. I've been living in the KarlNap tag on AO3 for months. The search filters there are your lifeline—sort by kudos or bookmarks, then dive into the ones with tons of comments; that's usually where the real heat is. Some authors have mastered that specific, feral energy Karl and Sapnap have in the monster AU tags.
Don't sleep on Tumblr either. The tag is a mess sometimes, but folks will reblog gems with really specific commentary. I found this one epic, multi-chapter fic through a Tumblr thread that wasn't even tagged properly on AO3. Twitter's trickier, but following artists who draw Monster Ultra fanart often leads to fic rec threads. It's a hunt, but the payoff is those stories that nail their dynamic—the competitive bickering laced with this weird, codependent affection.
Archive of Our Own is absolutely the backbone for quality and organization. Once you find an author you like, check their bookmarks; that's a goldmine of similar tastes.