I don't think after spending so many years writing with a friend 83490434092 versions of "what if Rui had left for Arc Land and had to simmer before things go awry enough for him to get the push to talk it out with them (and realize how stupid the entire thing has been because their hearts were always same all this time)" even before WL2 was ever a thing, I can say anything else is my favourite atp.

Though that feels like a bit of a cheat answer, because that's multiple verses (including some pretty major differences like whether if there is another sekai/some fragment/even how long it ends up being and how before things hit a breaking point).

If you had meant card based ones though, I suppose mainly two different verses shaped after shibufes Rui card comes to mind, one that was born from overthinking some details from Princess Tutu with a friend, and other was very loosely me thinking of how fun and awful putting Rui in anything Gravity Falls' Transcendence AU adjacent situation would be, mainly to tap into his fears of being a monster like people made him feel like, far before Rui 5 ever was on the horizon.

(Funfact, that latter verse even had a moment between Rui and Luka at night, where she comforts him with letting him rest his head on her lap and sings him a lullaby, after he ends up talking about how he's unsure what to do next about his current situation, as he having trouble sleeping for "can't really sleep as not alive not dead but secret third thing existence" reasons, with them staying the night in the sekai bc it was the only place Emunenekasa could interact with him easily without needing to be asleep themselves at the time.

^ As you can imagine, I had a real ball with that verse in terms of playing around with things like how a '(partial) dream demon' would interact with a world made out of magitech-based-on-music-made-of-strong-feelings, one specifically made of positive ones on top of that, just how his presence if tapping into TAU at all would effect WxS Sekai, and ditto in the future even when Emunenekasa eventually pass away, wouldn't he be able to keep some part of it alive, though not unchanged, if their sekai became a part of his mindscape, etc.)