In Shakespeare's Macbeth, I think the most important symbolic scenes start with the "dagger" scene where Macbeth is trying to stir up his courage to kill Duncan. This shows what a good man Macbeth is at the start of the play: a brave, honorable and loving servant of the King.
Another symbolic scene is when Lady Macbeth turns on her husband and questions his manhood because he has second thoughts about killing the King. This is symbolic because it alludes to the true nature of the this married couple. It will also be significant to note in the last act, how much Lady Macbeth has changed from the cold, calculating woman we see at the play's start, to the broken woman, crippled by her guilt, at the end.
Macbeth's hallucinations after killing Banquo are symbolic of good vs evil: of honor vs treachery. And when Macbeth sees the image of Banquo's "issue" (descendants) traveling down the through the ages, Macbeth's reaction is symbolic of how deep his treachery lies. The fact that Banquo's sons will be kings drives Macbeth mad, even though he is now the King of Scotland.
The scene where Macbeth returns to the witches for new predictions is symbolic of his overblown ambition and his "false sense of security." In this scene we are able to see how much Macbeth has deteriorated over time, and how foolish he is to believe that he can command the powers of darkness. Macbeth should know better, but he ignores what he knows about those who serve the Devil.
Finally, Lady Macbeth's sleepwalking scene is symbolic of the guilt she feels for not only her part in Duncan's murder, but her guilt for what her husband has done since then, of which she has had no part. It is clear that evil acts cannot help but touch the fragile psyche, and Lady Macbeth goes insane.
In my opinion, these scenes are some of the most symbolic in the play.