Been hunting for Hachiman x Totsuka stuff for a while, and my top spot is Archive of Our Own, hands down. It's got the most volume and, crucially, the best tagging system—you can really drill down into 'SaiKi' or 'Saika Totsuka/Hachiman Hikigaya' and filter by word count or completion status. That's vital because the pairing has a lot of short, fluffy one-shots mixed with some genuinely ambitious longer works, and AO3 lets you sort that out. The quality varies wildly, but the tags mean you usually know what you're getting into, whether it's sickeningly sweet domestic fluff or something that leans into the ambiguity of their dynamic.
I've found some real gems on FanFiction.net too, but it's a bit of a different scene. The search is clunkier, so you end up scrolling through a lot of general 'Oregairu' fics. When you find a dedicated Hachitan story there, though, they often have a more classic fanfic feel—some authors who've been around for a decade and have a very particular, dialogue-heavy style. It feels a bit more nostalgic, maybe? But the downside is you'll also find more abandoned WIPs.
Honestly, the 'best' is subjective. If you want polished prose and complex explorations, AO3's curation through kudos and bookmarks is unbeatable. For a raw, unfiltered archive that captures the fandom's history, FFN has its place. I sometimes cross-post my own stuff to both, just to see where the conversation goes—AO3 readers tend to leave more detailed comments dissecting character motivation, which is perfect for a pairing as layered as this.