Ah, Ophelia. She is an enigma. We know that Ophelia affects other characters profoundly, but during the entire play, Ophelia has only one soliloquy, so we are not privy to her thoughts. We see her mainly as others see her, and she becomes the victim of the corruption within in the court. Her death by water rather than poison is perhaps indicative of her innocent nature.
But if I had to choose one quote to some her up, I would look at her one soliloquy:
O woe is me
T'have seen what I have seen, see what I see!
Caught between obedience to her father and her love for Hamlet, Ophelia chooses her father. In so doing, she witnesses Hamlet's pretend insanity and later his intense anger toward her. His frustration with her choice and with his situation in general causes him to lash out at her, repeatedly telling her to "Get thee to a nunnery."
As if the change in Hamlet is not enough, Ophelia learns that the man she once loved killed her father. It is a tragic event that she cannot handle. It completely breaks her.
Even Claudius feels remorse when he sees the mad Ophelia,
When sorrows come they come not in single spies, but in battalions.
Ophelia witnesses her transformed lover turn into a madman and her father murdered at his hands. It is no wonder that she goes mad and slips into the brook and submits to its current. Her death is a passive acceptance of the currents that are much too strong for her. I think even Hamlet recognizes that Ophelia could not withstand the corrupted currents of the court when he advised her to go to a nunnery. But by Act 4, Ophelia becomes the victim of the struggles of others: she has witnessed too much and what she has seen literally and figuratively drowns her.
Well, you might want to start off by considering more generally what kind of character Ophelia is and the kind of role she plays in the play. "The fair Ophelia" is distinct from other Shakespearian heroines because she is passive almost to the point of non-existence as an independent consciousness. Consider her uncomplicated obedience to her father and brother in their instructions to watch out for Hamlet. Many other Shakespearian heroines put their love for their prospective husbands before the wishes of their father, and let's face it, Polonius isn't the most terrifying father-figure Shakespeare has created. Ophelia doesn't actually struggle much against what is happening to her. She is not really developed as a character in the play, and she is presented as someone with no point of view. It is only after the "nunnery" episode in Act III scene 1 that Ophelia speaks to the audience and she presents herself as entirely a victim of Hamlet.
Perhaps, then, given this information, a good quote to sum Ophelia up might be what she says in Act I scene 3:
I do not know my lord what I should think.
This sums her up as it shows she is not able to think for herself - she is not able to act by herself and she is not able to make decisions by herself. She is a passive character whose worth is only to highlight the characteristics of others.