Which soliloquy in Macbeth best summarises the themes of the play?

There are of course a number of soliloquies that could be selected as being the soliloquy that best captures the themes of this play, but arguably, Macbeth's soliloquy in Act III scene 1 after he has bidded farewell to Banquo and Fleance is the soliloquy that captures best the overriding ambition of Macbeth, which is of course one of the central themes of the play. He recognises in this soliloquy that Banquo still represents a major threat to him, not just because he is the only witness to the prophecies that the witches gave Macbeth, but also because Banquo himself received a prophecy that remains deeply troubling to Macbeth, as they predicted that Banquo would father a line of kings. Macbeth finds this a terrible position to be in:

Upon my head they plac'd a fruitless crown,

And put a barren sceptre in my gripe,

Thence to be wrench'd with an unlineal hand,

No son of mine succeeding.

The prophecy of the witches, whilst it has given Macbeth the crown, has rendered that crown "fruitless" by the prophecy that it will be Banquo's heirs who will possess that crown after Macbeth. The trappings of power in this quote, the crown and sceptre, are described in ways that makes them impotent, as the sceptre is "barren." Even though Macbeth on the one hand now has what he wanted, and was prepared to kill to get, now he finds that he only wants more. This of course is the way that ambition works: having satiated his hunger for ambition through killing Duncan, Macbeth finds that it is not enough, and he needs to continue to kill and commit acts of evil to satisfy his never-ending hunger for power.