Obviously there are no hard and fast answers to this question and everyone is going to differ in their response. What you need to think about is how Macbeth's character develops - how we see him at his first introduction in Act I scene iii, and then what are the significant stages that lead him up to his tragic downfall at the end of the play. Where are those stepping stones in his character development? Here are my ideas:
1) Obviously Act I scene iii is key in our first introduction to the character of Macbeth. This is also a key scene because the witches give Macbeth their prophecies and we see how Macbeth reacts. Disturbingly, the text suggests that the withces have actually keyed into thoughts that Macbeth has already contemplated, and also introduce a key issue as to whether Macbeth himself needs to act to accomplish these prophesies.
2) Act I scene vii is another key point as we see Macbeth's conscience in his soliloquy, but also how he is bullied into committing the murder by Lady Macbeth. We see her cajole, mock and flatter him into committing the murder and this makes Macbeth "resolute" as he goes off to murder Duncan.
3) Act III scene i to me is important because, with Macbeth's organising of the murder of Banquo, we see a Macbeth who is able to commit acts of villainy without the involvement of his wife. Also, hiring murderers to do the deed for him instead of committing the murder himself raises his crime to new levels - he is orchestrating slaughter and violence from a distance, and showing greater savagery.
4) Act IV scene i and ii represents an escalation of what has been established through the murder of Banquo. Note how Act IV scene ii is deliberately designed to evoke the full horror of Macbeth's actions - Macduff's family become real, sympathetic people who we can empathise with and this makes Macbeth's crime all the more heinous. Macbeth is well and truly on the slide to damnation.
5) Act V scene v, one of Macbeth's last scenes, is particularly tragic in the way that Macbeth greets the news of his wife's death and also realises that his time is up. What makes it more poignant is the manner in which he reacts to these events. His determination to die fighting and not to give in adds much to the tragedy of the play - Macbeth has many noble qualities such as bravery and courage, and it is said how he has fallen.