The 'best' is such a subjective thing, isn't it? Depends entirely on what you're craving. For sheer volume and a decent tagging system, Archive of Our Own is unbeatable. The MDZS fandom is massive there. You can filter by ship, by trope, by completion status... it's a lifesaver when you're hunting for a specific flavor of WangXian angst.
That said, sometimes I find more passionate, niche works on smaller, dedicated forums or on Twitter threads. Those can feel more raw and immediate, like you're reading someone's secret diary entry about these characters. The polish might be missing, but the heart is overwhelming. Don't sleep on Weibo if you can navigate it, though that's a whole other ecosystem.