Which of Hamlet's seven soliloquies is most important to the development of both the Hamlet's character and the plot?
The most famous soliloquy is the most important for Hamlet's development. "To be or not to be, that is the question . . ." is from his famed speech in Act 3, and it expounds on Hamlet's character, his decisions, his motivations, and his eventual actions. It is in this scene that Hamlet resigns himself to the fact that he may not succeed in his journey; he may in fact die. And he ponders: what is it to die? Is it like sleep, to go off and dream? Or is it more?
His musings in this speech resolve him towards the tasks at hand and, coming just prior to the climax of the play, help to steel him for the battle that is brewing. He gathers his strength and realizes that he must go forward with his plan to kill Claudius and Laertes, an act of vengeance to honor his father, but he knows with it comes near-certain death. It is for this end that he resolves himself and contemplates the meaning of life and death.
I think the first soliloquy Hamlet delivers, in act 1, scene 2, is the most important in terms of establishing his character and laying the groundwork for the rest of the play's plot. We have already seen Hamlet's lackluster interaction with his mother and stepfather/uncle; we've seen Claudius rather coldly deny Hamlet's request to return to Wittenberg, as well as his friendliness and warmth toward Laertes, the son of Polonius. In the soliloquy that follows, we learn of Hamlet's desire for death, his feelings about the state of Denmark, his grief for his beloved father and his anger at his mother's betrayal and hasty remarriage to her brother-in-law, in addition to his feelings about women's frailty in general. This soliloquy helps us to understand Hamlet's feelings about a great many subjects having to do with his parents as well as his temptation to take his life were it not for God's rules against it. All of this information is crucial for our later understanding of his actions and feelings.
Hamlet's most important soliloquy in terms of character and plot development is in Act 3.1.64-99.
In these verses lie the most important philosophical and moral quandaries with which Hamlet (and indeed, humanity) struggles: what is the point of living? Of being? Is it better to endure the hard times, knowing that there is no end to hard times or end life yourself? Why do we struggle with conscience (and consciousness?)
As Hamlet discovers, there will be no easy, or even satisfactory, answers to any of his questions. He determines his character, and the unfolding of the plot, by wrestling with each one of these dilemmas.
Hamlet's internal turmoil has resonated with playgoers and readers for centuries. Some of the most familiar quotations in all of Shakespeare are found here: "To be or not to be -- that is the question"; "To die, to sleep --/To sleep perchance to dream"; " when we have shuffled off this immortal coil";"conscience doth make cowards of us all."
... To die: to sleep;
No more; and by a sleep to say we end
The heart-ache and the thousand natural shocks
That flesh is heir to, 'tis a consummation
Devoutly to be wish'd. To die, to sleep;
To sleep: perchance to dream: ay, there's the rub;
For in that sleep of death what dreams may come
When we have shuffled off this mortal coil,
Must give us pause: ...
im my opinion it is his 7th. it ties the whole play together. its hard to put a "most important" mark on something like that though