Arguments about being the most powerful are pretty much stupid, due to the spellcard rules... And any other clue we have is just not enough to clearly draw the line...
RiderFan said: Yuuka is the most powerful, but she just casually goes about her business, even in games.
And Yuugi can control supernatural phenomena, that's just about everything in Gensokyo by the way. If interpreted right she can make whoever she wants powerless, or gone. Yuugi is boss.