Navigating that space always feels like a particular kind of quest. I'm not sure there's one definitive 'best' site; it really depends on what flavor you're after and how you like to read. My initial go-to was always Archive of Our Own—the tagging system is unmatched, and you can filter for the specific projects he's been in, like 'Dune: Part Two' or 'Elvis', which helps a ton. But honestly, Wattpad has a different vibe; it's where I found some surprisingly sweet, character-driven one-shots that felt more like missing scenes. Tumblr is a whole other beast, great for drabbles and moodboards that inspire fics, but the actual stories can be scattered across different blogs, which is its own scavenger hunt.
I'd say start with AO3, get a feel for the tags and pairings that appeal to you—maybe you're into his character from 'The Shannara Chronicles' or curious about crossovers. The quality can vary wildly, but the kudos/bookmarks system usually surfaces the standout works. Wattpad feels a bit more hit-or-miss in terms of polish, but sometimes the raw, enthusiastic ideas there are just fun. I still check both, depending on my mood.