Okay, folks, I had a few ideas for this, so I had to consult a trusted opinion to make a final decision.
What is your favorite crack pairing?
For the purposes of this question, "crack pairing" will be defined as two Touhous that don't get shipped very often, possibly because they've interacted with each other very little or not at all. For example, things like Reimu x Yuuka or Sakuya x Eirin, while unusual, aren't crack pairings per se because they've interacted in a notable fashion and can be found pretty easily if you know where to look. Things like Yuyuko x Byakuren or Elly x Komachi would qualify despite obvious character connections, however.
I'll edit mine into this post once I've figured it out - I want to get the question out there. (Believe it or not, my answer is not Aya x Reisen.)
EDIT: I've decided on Reisen x Sanae because it has the potential to range from utterly adorable to smoking hot while hitting everything in between. I am imagining Sanae petting Reisen in my head and it makes me feel fuzzy.