I choose the moment when Macbeth changes his attitude about the possibility of being king from passive to active. Shortly after the witches' pronouncement, Macbeth says:
If chance will have me king, why, chance may crown me,
Without my stir.
Then, not so very long later, he is in the presence of the King; Duncan has just named Malcolm as his successor. At this news, Macbeth says:
It's clearly this moment when the audience must realize that Macbeth will be taking action to make the prediction become a reality.
This is one of those wonderful questions where pretty much anything goes as long as you can justify it, so I am going to disagree with my learned e-colleagues above and state that for me Act I scene i is the most important scene of the play
This is because it completely sets up the setting and foreshadows much of the action - the blasted heath strikes the appropriate sombre tone and the key line "fair is foul and foul is fair" is something that foreshadows many aspects of the plot to come. It also introduces the witches as key characters in the play, emphasising the role of black sorcery and posing an intriguing question: does Macbeth do what he does because of the witches or would he have done it anyway?
I believe that Act II, Scene 1 is important in foreshadowing Macbeth's mental collapse later in the play; however, the most important scene should be the scene that expresses the most conflicting events. Act 3, Scene 4 is the most important scene in the play because Macbeth is at the high point of his reign, and ironically it is the beginning of his downfall. Shakespeare strategically writes a most interesting scene where Macbeth is receiving guests at his banquet as King, and than moments later is in distress upon seeing Banquo's ghost! You can see the play unravelling before your eyes.
I agree that the most important scene in terms of plot and character development is the first scene in Act II when Macbeth hallucinates the dagger. The dagger is a symbol of Macbeth's guilt and fear and it is at this moment when he questions and challenges his fate while looking for the right decision to make. This is Macbeth's "crossroads," the point at which he must decide which path to travel.
I don't necessarily disagree with Post #2, but I think you could also consider Act II, Scene 1. I don't think the witches force Macbeth to kill Duncan -- I think he is at fault. So this scene is important because it is where he actually decides to do the deed that will bring his doom.
The most important scene in Macbeth is the scene in which the three hags inform Macbeth (to paraphrase their words) that he will be King. This is the match that set of the conflagration, that led to Lady Macbeth’s plote, that led to the murder of the King:D
Please when answering questions, be exact and answer it all!
Thank you very much to all of you guys!