Actually, 'best' is pretty subjective depending on what you're looking for. AO3 absolutely has the highest volume and the most advanced tagging system, which is a lifesaver if you want to filter for specific dynamics or exclude certain tropes. The quality tends to be higher there on average, in my view. But it really depends on your tolerance for sifting through stuff.
That said, I sometimes find the real niche, character-specific gems on Tumblr or even Quotev. They feel less polished but way more personal, like someone wrote it just for their own little corner of the internet. Wattpad's algorithm is a mess, but if you manage to find a good writer, their whole catalogue can keep you busy for a week. Archive of Our Own is still the main library, though.