Another excellent virtuoso part for Lady Macbeth is in the famous sleepwalking scene in Act 5, Scene 1. Lady Macbeth is reliving the murder of Duncan and revealing her feelings with both her dialogue and her actions. The Doctor and a Gentlewoman are secretly observing her and making comments on her behavior which provide additional information to the viewer, including the fact that this sleepwalking is something Lady Macbeth has been doing for a long time. She is carrying a single taper, so the stage would be eerily lighted, contributing to the effect of the entire scene. Her guilt-haunted manner and spoken confessions would stand in sharp contrast to her ambitious, self-confident wickedness in most of the previous scenes in the play. Presumably she was evil but beautiful in the earlier scenes but would look old and haggard in Act 5, Scene 1.
Clearly you need a section of the play where we see Lady Macbeth in all of her determined passion and where her resolute and strong nature is revealed. For me, the best soliloquy that she delivers that would be perfect for these purposes would be in Act I scene 5, when she receives a letter from her husband telling her about the prophecies of the witches and she prays to the various evil spirits to help commit herself on her path of evil to make sure her husband receives the prophecy that the witches refer to:
Come, you spirits
That tend on mortal thoughts, unsex me here,
And fill me from the crown to the toe top-full
Of direst cruelty! make thick my blood;
Stop up the access and passage to remorse,
That no compunctious visitings of nature
Shake my fell purpose, nor keep peace between
The effect and it! Come to my woman's breasts,
And take my milk for gall, you murdering ministers,
Wherever in your sightless substances
You wait on nature's mischief! Come, thick night,
And pall thee in the dunnest smoke of hell,
That my keen knife see not the wound it makes,
Nor heaven peep through the blanket of the dark,
To cry 'Hold, hold!'
This is of course a key speech, as it demands a range of emotion and excellent acting abilities. In this speech we can see how Lady Macbeth is so ambitious, both for herself and her husband, that she willingly asks the evil spirits to possess her, body and soul, and makes something of a Faustian pact with them in exchange for the evil character she feels she needs to see her husband become King through the death of Duncan. This would be an excellent speech to deliver.