The rules sticky for this board outlines why power level discussions are verboten around here. As I think about that particular rule, I think that since it comes up often enough, it might - MIGHT - be a good idea to have a separate "let's all GRAR about power levels" thread, where those who want to engage in the futile and ultimately unresolvable discussion of power levels can do so, strictly within the confines of that thread, and everyone else who's sick of such discussions can stay away from it. Sort of like quarantining people who have some disgusting contagious illness, while everyone else goes about their daily lives. A power levels leper colony, if you will.
Anyway, that's for a mod to decide.
In answer to your question though - since it is answerable - canonically speaking, ZUN has never sat down and drawn up a power levels chart of Touhou characters. If you asked him who the strongest really was, he'd likely laugh at you. As it is, there isn't even any quantifiable data on how powerful a particular Gensokyan is; only what their power is.