This seems kind of like a bad trick question. Vanilla answerers would say its the dom, and I have heard several times that its the sub. My mistress, who's also a submissive, says the submissive has the power because I can walk away at any time, but that only gives me one single power I don't want to use. I could attempt to blackmail her into making the changes to our relationship that I want, but that's dumb and petty and wrong. Also, in my case, and I'm sure in many others, it wouldn't work. Shed just tell me to fuck off.
Other than that, I attempt to express feelings that conflict with hers, she gets offended, and no discussion occurs. She's just mad at me.
Anyway, this "power" that I've been told I have is a power everyone in every relationship has, and its not very helpful. In a normal (I didn't say vanilla), healthy, monogamous relationship, I would say each partner has equal power.