I keep seeing people recommend AO3 for Sans/Papyrus stuff, and I'll admit the tagging system is a beast, but honestly? The quality's a total lottery there sometimes. A huge chunk of it is just... not very good, or it's those weird hyper-specific kink memes spun out into 500 words. Tumblr blogs that are still active often have better curated rec lists in their pinned posts, from authors who've moved off the site but left their work up. You have to dig through a lot of old reblog chains, though.
Some of the most interesting takes I've found were actually on smaller, fandom-specific forums that popped up after the game's hype died down. The character exploration felt less constrained by popular fanon tropes, like the whole 'brotherly love turned awkward' angle everyone recycles. I found this one series that framed their dynamic through the lens of shared responsibility for the Ruins, which was a fresh spin.
Honestly, my best finds lately have been from following individual authors who cross-post between Pillowfort and Dreamwidth, weirdly. The community's smaller, so the writing tends to be more deliberate.
Honestly? This is gonna sound weird coming from someone who's been in the Undertale fandom since the game dropped, but I'm gonna have to give it to Quotev of all places. Not even joking. AO3 is the obvious king for sheer volume and tagging, and yeah, you'll find incredible, polished novel-length fics there, especially for intricate AUs. But the Sans x Pap stuff on Quotev has this... raw, unfiltered energy sometimes. It's where a lot of younger writers or people just vibing with the ship first start posting, and you get these wild, unapologetically self-indulgent one-shots that don't always follow conventional narrative structure. They're just pure id, you know? The prose might be rougher, but the emotional hits can be startlingly direct.
Don't get me wrong, you absolutely need AO3 in your life for the heavy hitters—the psychological deep dives, the meticulously plotted stories where their dynamic is explored through every possible lens, from horror to domestic fluff. But if you want to feel the pulse of the ship's more immediate, fannish heart, Quotev's search and filter system is clunky as hell, but digging through it can unearth gems you'd never find otherwise. Tumblr's a whole other beast; it's more for headcanons, drabbles, and art with snippets, but the reblog chains can lead you to fic threads that never get cross-posted. My personal bookmark folder is a messy mix of all three, honestly.