Quite frankly, none.
To be honest pretty much every single fan pairing is loosely based upon characters interacting with each other or having a friendship with each other. It’s not surprising that this happens as it’s not just Touhou who get fan pairings made from friendship. Take Marisa and Alice for example there’s no dialogue to implicate they’re a couple, but rather their mutual interest in magic has developed into a friendship. Remilia and Sakuya, well it’s a master/servant relation… I could go on…
Have a picture of Reimu instead, I know you’ll like it~
Without a doubt, the Perfect and Elegant Maid of the Scarlet Devil Mansion Sakuya Izayoi, manipulator of time and servant to The Eternal Scarlet Devil. Time manipulation works so well for her, allowing her to perform extraordinary duties in the Scarlet Devil Mansion, she’s even managed to slow down her own ageing process as well.
The God and creator of Makai, Shinki-sama.
A God with the sheer power to create an entire realm along with it’s inhabitants is a remarkable ability. Sure there are many fun and rather unusual powers that many Touhou’s poses, but very few are on the level of a God. Not to mention she has a wonderful maid at her side who rather than stabbing you with a knife, uses entire swords! It’s probably best not to anger or invaid Makai for your own safety.
To fit best in the outside world, the Touhou in question must first be someone who wouldn’t cause an incident and would leave peacefully with humans. To this extent there are very few to choose from. I would have said Sanae as she was originally from the outside world, but she probably wouldn’t be able to cope any more. Byakuren could have been a good choice, but she’d most likely cause a religion war thus starting World War Three.
With that in mind I rather bizarrely choose Genji! The old lovable turtle Reimu used to ride into battle, he does very little these days but relax in the lake behind the shrine. A peaceful turtle would fit perfectly into the world as no human would disturb him and he could easily escape or literally fly off if someone bothers him.
Fit worst in the outside world? Well, wouldn’t it simply be all of them? So many powerful Touhou would just get bored in the outside world let alone those who might want to go for a bit of chaos and have no body around to stop them.
Hmmmm.. Ahh I know, Sariel, the angel of death! Well at least in theory she could be quite dangerous, there isn’t really an exchange of dialogue to go with, par her title and music theme being “Now, until the moment you die” & “Death will take you with me”.
Without a doubt the most underrated Touhou’s are PC98 characters, but the little immolating black witch from mystic square is my favourite.
In Mystic Square she appears to be good friends with Mai, who unusually seems like a polar opposite as she uses ice magic and wearing white. If you happen to defeat Mai first in Mystic Square when they attack you Yuki will be come enraged.
You could see Yuki as being a character who loves a bit of pyromania who’s only kept in check by Mai. There could be a palava of comedic sketches for these two with Yuki causing a loveable spot of fiery havoc.