PS: This question can have several answers depending on how we define "Best". Whether it's in exorcist power, in combat strength, or in reasoning...
I chose Kō because in my opinion, even if he's not the "strongest", he doesn't exorcise spirits just because they are, and he protects the kind spirits.
Even if it may seem naive and that he can easily be fooled, I find that it's very wise of him, and it's surprising that Kō is the one who reasons the most...
All this reminds me of the Age of "Lumières" (Enlightenment), I learned it in history at school and I didn't expect to think about it again today...
Anyway, that's my opinion. What do you think?