If you're after that specific brand of emotional carnage and snarky devotion, Archive of Our Own is basically the main hub now. The tag system is a lifesaver for sorting through the mountain of content. You can filter by 'Spike/Buffy Summers', then sort by kudos or bookmarks to see what's standing the test of time. I'd also suggest looking at the collections people have made; some users curate 'best of' lists for the pairing, which saves you from wading through the less polished stuff. Don't just stop at the first page of results either—some truly fantastic authors aren't as prolific, so their stories might have fewer kudos but are absolute masterpieces in character voice.
A lot of the classic, epic-length stuff from the early 2000s migrated over from sites like The Bloodshedverse or Elysian Fields, which are still around but much quieter. It's worth a search on those for the foundational texts, so to speak. The writing style on those older archives can feel different—more novelistic, sometimes less constrained by the tagging culture of AO3. For a different flavor, check out specific communities on Dreamwidth or even certain corners of Tumblr where writers post snippets and link to their full works. The search is part of the fun, honestly; you never know when you'll stumble on a 300k word post-'Chosen' fix-it that completely rewires your brain.