Ambition, power, greed, guilt are all themes of Macbeth. But I think the one theme that stands out above the others and the one idea that separates Macbeth from many other works that share this same theme is the way Shakespeare shows us the repercussions of guilt and the ways in which it can deteriorate our psychological wellness and make us "go mad." We see this in many of the characters throughout the play, but we see it most clearly in Macbeth and Lady Macbeth. Macbeth's reaction to the death of Banquo and the effects of Lady Macbeth "un-sexing" herself into an embodiment of evil only to come away from it being nothing more than the hollow shell of a human being are among the greatest expressions of this idea in all of literature.
There are many, but I would say that the two most important are ambition and fate or destiny.
The theme of ambition plays out through more or less the whole play. It is Macbeth's ambitions (added to those of his wife) that (you can argue) make him do all the things that he does in this play.
The second theme is brought up by the witches. This theme explores how much people's lives are defined by fate and how much is within people's own control. Do the witches' prophecies cause Macbeth to do what he does or does he choose to do these things on his own?
Another theme is the idea of guilt, often of course brought on by the other themes of ambition and drive for power, particularly in the case of Macbeth himself.
There is also the recurring theme of the supernatural. The combination of the witches with their ability to influence events and the way that fate seems to twist around to come back and get those who try to change it is an important part of the play.