One could justify that the witches in Mabeth are the most powerful characters in the play. Without the witches, Macbeth would have not heard the prophecies, desired the throne so badly (as a result of his raised ambitious nature), murdered Duncan and Banquo, and died at the hands of Macduff.
Also, given that Macbeth returns to the witches later in the play, to find out more information, shows that he even identifies the witches as powerful.
Not only do the witches have power over the happenings of the play, they also have power over the setting and mood of the play. The play opens with the witches. Thunder and lightening are seen and heard. Normally, the opening of a scene shows some of the most important characters (or most powerful). Given that the play opens with them, the witches can be seen as being important enough (or powerful enough) to open the play with.
In Macbeth, the witches are the most powerful characters because of their ability to influence Macbeth. To provide an example of this, consider Macbeth before he meets with the witches. His loyalty to King Duncan is without a doubt: he risks his life for Duncan on the battlefield, for instance, and is already the Thane of Glamis, a title which further highlights his loyalty to and special relationship with the king.
Once Macbeth meets with the witches and hears their prophecies, Macbeth's character and values begin to rapidly change. When Duncan makes him the Thane of Cawdor, Macbeth perceives this act as proof that the witches are correct when, in reality, it is a token of Duncan's gratitude. Moreover, Macbeth immediately starts to think about murdering Duncan. He has clearly never had thoughts like this before since he is disgusted and frightened by them, as shown in Act I, Scene III:
Why do I yield to that suggestion
Whose horrid image doth unfix my hair
And make my seated heart knock at my ribs.
So, the witches play a pivotal role in eroding Macbeth's loyalty to Duncan and awakening his inner ambition. It is for these reasons that they are the most powerful characters in the play.
Withches are the most powerful characters in Macbeth because without them plot cannot be developed.