Man, picking 'best' tropes for Sebastian x reader fics is tough because it depends so much on what flavor of dynamic you're craving that day. I lean towards the ones where the reader has some kind of hidden power or lineage—maybe they're the last descendant of a rival demon clan or have a celestial blessing. That immediate throws a wrench into Sebastian's usual 'perfect demon butler' schtick. He can't just dismiss them as another fragile human plaything; there's a genuine, dangerous curiosity there. The slow-burn of him figuring out if they're a threat, an equal, or an exceptionally intriguing specimen while maintaining that impeccable facade is just... chef's kiss.
Another trope I keep circling back to is the 'Contractual Complications' scenario. It's not a standard master-servant contract; maybe the reader accidentally binds him through an archaic ritual, or their soul has some bizarre condition that makes consuming it problematic. The fun is in the loopholes and the strange domesticity that emerges. Sebastian still does everything with flawless, eerie precision, but there's this undercurrent of him being genuinely, if annoyed, invested in the puzzle. You get moments of him being almost protective, not out of affection, but because his 'property' or 'investment' is being threatened. It walks that perfect line between menace and a twisted, possessive care. I've read a few where the reader is another supernatural entity posing as a new staff member at the manor, and the constant, polite power plays over tea service and dusting are weirdly intense.
Honestly, I sometimes skip the outright romantic fluff for these more gothic, tension-heavy plots. The appeal of Sebastian is that otherworldly, unsettling control; the best fics make you feel the reader's spine tingle even as they're drawn in. A good one leaves you wondering who's really leading whom into the trap.