Through sheer force of will, Lady Macbeth seems to be the most powerful character in act 1. She is anxious for her husband to return home so she can control him. She even says,
Hie thee hither,
That I may pour my spirits in thine ear
And chastise with the valor of my tongue
All that impedes the from the golden round [...]. (1.5.28-31)
She wants to compel him to behave ruthlessly, remorselessly, compelling him to murder his king, cousin, and friend, so that the pair of them can ascend to the throne by the "nearest ways"—the fastest possible path—rather than waiting around for Duncan to die. In the final scene of this act, Macbeth decides that he will "proceed no further in this business" of regicide, and Lady Macbeth quickly overpowers him, insulting his manhood and pride by calling him "green and pale" and "a coward" until he gives in to her.
Lady Macbeth continues to hold the power in this relationship during act 2 as well. When Macbeth refuses to return the bloody daggers to Duncan's chamberlains, she berates him again, calling him "Infirm of purpose" and saying how easy it is to wash their crime away with a little water. She says,
My hands are of your color, but I shame
To wear a heart so white. (2.2.82-83)
Again, she insinuates that Macbeth is a coward and ought to be ashamed of his fearfulness and weakness.
In act 3, however, there is a subtle shift in power between these partners. Lady Macbeth feels only "doubtful joy" (3.2.9), and Macbeth says that his mind is "full of scorpions" (3.2.41). Neither is happy with their new state power—Macbeth, because he feels that Banquo is a threat to him, and Lady Macbeth, because a schism is developing between herself and her husband. Now, he "keep[s] alone" and makes big plans without telling her (3.2.10). He arranges for the murders of Banquo and Fleance, without her input or even her knowledge, and then he tells her how to act rather than she him (as was the case in act 1). By the end of this act, he is resigned to bloodshed while she seems a great deal more reticent. However, he informs her that they "are yet but young in deed" (3.4.176), and he begins to seem the more powerful of the two.
In act 4, as if to show that she has completely lost power in the relationship, Lady Macbeth is absent. In fact, the banquet scene of act 3 is the last time the audience actually sees the now king and queen together. In this act, Macbeth goes to the Weird Sisters for information and arranges for the murders of Macduff's wife and children, all without his wife's input.
Finally, in act 5, we see Lady Macbeth's famous sleepwalking scene. It seems that she has been overpowered by her guilt, imagining that she cannot wash Duncan's blood off her hands. She cries about how "much blood" was inside Duncan and alludes to the murders of Lady Macduff and her children (implying that their deaths, too, weigh upon her conscience). She seems utterly powerless at this point, while the state power passes to Malcolm from Macbeth. Macbeth's act of overwhelming dominance and cruelty, killing Macduff's family, compels Macduff to kill him, and thus the Scots throne passes to Malcolm. Ultimately, the only power Lady Macbeth retained was the power to take her own life, and Macbeth could only willingly fight his foe who he, himself, empowered.