Man, that's a tricky one because 'best' is super subjective. I've been lurking in that ship's corner of the internet for ages, and honestly, the landscape keeps shifting. Archive of Our Own is probably the most reliable for consistent quality and decent tagging—you can filter for explicit content and word count, which helps weed out the rushed stuff. But sometimes the real gems are tucked away on old, nearly-forgotten forums or personal blogs.
A few years back, I found this amazing, multi-chapter SasuHina story on a now-defunct Naruto-specific site that did the enemies-to-reluctant-allies-to-lovers trope perfectly. The lemons weren't just thrown in; they felt earned. I lost the link and I'm still mad about it. These days, I'd say dig around on Fanfiction.net too, but you gotta wade through a lot of... well, let's call it 'early-2000s writing style' to find the good ones. The search function is a nightmare.
Honestly, half the hunt is knowing which authors to follow. If you find one writer who nails their dynamic, check their favorites or bookmarks—they often lead you to other hidden treasures.