For the purposes of this thread, assume said Touhou character is balanced for League (so no OP shenanigans from you, Yukari). Kit not necessary, but if you can make a basic one, go right ahead.
My pick? Obvious. http://i85.photobucket.com/albums/k68/qasd24/Hakurei-Reimu.jpg
http://www.donmai.us/data/sample/sample-7e241eb07889b7827675f793dc16bb9e.jpg One of my first thoughts is Patchouli Knowledge. I have two kit ideas. One is the ultimate creates one spell based on the two previously cast spells. The other is along the lines of each spell acting like Karma's Mantra, modifying the next spell cast.
As for Yukari Yakumo, her Ult summons a train.
Why am I not saying Cirno? Not much she can do that 
haven't done already.
Did you know that the design of Lux is based on Marisa from Touhou? Two of her skins are even references to Marisa, and Lux's ultimate looks a lot like one of Marisa's bomb spells.
http://en.touhouwiki.net/images/thumb/d/d2/Th13Kogasa.png/200px-Th13Kogasa.png Kogasa Tatara with a jungle kit. "Surprise!" gank An enemy has been slain.
Damn it, I was gonna pick Reimu. Gorgeous as fuck! Aw well, for my secondary. . . oh wait. . . there's 
. . . nevermind. . .
This is 10 months old, but I suppose I would most like Sakuya to be included. Time freeze could be an AoE ult, with obvious limitations.
I think I have to go with someone from the Scarlet Devil Mansion, as they are the only ones that I really know anything about.
Out of that selection, I think I have to cast my vote for Sakuya Izayoi.
