Well, if we're talking about stories for Lilith and Lucifer from stuff like 'Supernatural' or 'Lucifer' the show or just general mythology, I've been poking around for years. Archive of Our Own is absolutely the powerhouse, no contest. The tagging system means you can filter for exactly the pairing you want, and there's a ton of deep, character-driven stuff there. You'll find everything from canon-divergent epics set in 'Supernatural' to softer, more romantic takes inspired by the TV show. The writers on AO3 tend to be really meticulous about lore, which I appreciate.
That said, don't sleep on smaller, fandom-specific forums. For 'Supernatural' especially, there used to be these dedicated LiveJournal communities back in the day, and a lot of that work got mirrored or inspired current writers. You can sometimes find rec lists on Tumblr that point to gems hosted on personal websites. FanFiction.net has some older material, but the search is a pain and the content can be hit-or-miss, with simpler prose. Wattpad feels geared toward a younger audience, so the themes might not be as mature.
Honestly, the 'best' is subjective. If you want complex, novel-length explorations of their dynamic as equals and rebels, AO3 is your spot. If you're nostalgic for the vibe of early 2000s fandom, chasing down those older archives via links from veteran fans might uncover some raw, passionate stories you won't find anywhere else. I stumbled on a fantastic series last year that was basically a political marriage AU, hosted on some writer's NeoCities page, and it blew my mind.