Well, it depends on what you mean by power really!
If we are to take power in a social context, we clearly recognise that Shakespeare was writing at a time when men had a lot more power and priveleges in society than women: they had the power of owning property, the right to education, the right to vote. Women did not.
Within the play, the most obvious male - female conflict would be Juliet and Lord Capulet: the young daughter and the male head of the household. If you refer to Act 3 Scene 5 where Juliet declines to marry Paris, you can see the power dynamic. Capulet clearly views her as his property to dispose of as he wishes: "an you are mine, I will give you to my friend". In the face of her continued refusal, he resorts to threatening to disown her of her inheritance and property leaving her to "hang beg starve, die in the streets, / For by my soul, I'll ne'er acknowledge thee". Indeed, he reverts to the most basic power imbalance between men and women: his physical strength. He threatens to "drag thee" to church and states that his "fingers itch" which implies that he is on the verge of physically striking her. This is certainly the way in which most modern productions present them.
One might ask whether Juliet, however, is so powerless. She has Romeo; she has the power to take or withhold her own life which she threatens and finally achieves; she has the power to choose where and when to relinquish her virginity, which she does to Romeo literally moments before this scene.
One could consider Lady Capulet to be a more powerless character than her own daughter. She is bound to Capulet more completely than Juliet; she has no alternative; she has not the strength to stand up to her husband. Admittedly this is reading between the lines an awful lot but a production of the play could develop this.
Tybalt could be another potentially powerful character: he is a male, aggressive and combative. However, again, Capulet overshadows him. At the masque in Act 1 Scene 5, Tybalt seeks to kill Romeo but his uncle forbids it saying "Am I the Master here or you?" It is, perhaps, this tension and rivalry between them that may lead to the fight with Romeo in Act 3 Scene 1 as much as his anger at the Capulets or his inherent antagonism to the word "Peace" which we learn from Act 1 Scene 1.
The nurse may be an interesting character to consider too: as one of the lowest class characters, and female, she is utterly powerless socially but her character has an enduring appeal in her earthy bawdy character and her genuine concern for Juliet. Her betrayal of Juliet when she suggest she marry Paris in Act 3 Scene 5 is perhaps more hurtful than Capulet's.
I we were to look at moral power, you might come up with a very different discussion: perhaps women and particularly Juliet show a moral integrity, strength and power that men lack?
Anyway, if we were to be looking at 3 characters that show differences in power between men and women in society, I would select Capulet, Juliet and Lady Capulet.