Elizabeth's glorious rejection of Darcy's first marriage proposal. Darcy, up until this point, has done nothing but acted like a total snot, and Elizabeth calls him out on it. Thus begins Character Development on Darcy's part, turning him into the guy that millions of women still swoon over.
Darcy laying the smackdown on a snide Caroline Bingley when she insults Elizabeth, informing her in the iciest manner imaginable that while he merely considered Elizabeth 'pretty' when he first knew her, it "has been many months" since he has "considered [Elizabeth] one of the handsomest women of [his] acquaintance." He leaves no room for doubt that Caroline herself doesn't share the honour.
Miss Bingley: I believe you thought her rather pretty at one time.
Mr. Darcy:(who could contain himself no longer) Yes, but THAT was only when I first saw her, for it is many months since I have considered her as one of the handsomest women of my acquaintance.
The way Elizabeth handles Lady Catherine when the latter confronts her on the rumors that Elizabeth and Darcy are engaged. In particular, Elizabeth's response to Lady Catherine's snobbish objection that she would be "quitting her sphere in which you have been brought up.''
Elizabeth: In marrying your nephew, I should not consider myself as quitting that sphere. He is a gentleman; I am a gentleman's daughter; so far we are equal.
Elizabeth's tranquilly furioustelling off of Lady Catherine, when the latter essentially tries to bully her out of an engagement (which doesn't, at that point, actually exist) to Mr. Darcy. Having previously unnerved the woman in Kent by making it clear she does not worship her like just about everyone else who knows her, she now defends her right to marry whomever she pleases, and basically tells her to "Mind your own business!" Go, Elizabeth!
Lady Catherine: You are then resolved to have him?
Elizabeth: I have said no such thing. I am only resolved to act in that manner, which will, in my own opinion, constitute my happiness, without reference to YOU, or to any person so wholly unconnected with me.
Reader: GO ELIZABETH!
Mr. Darcy's selfless heroic rescue of Lydia's — and, therefore, the whole Bennett family's — honor, which involves negotiation to his own financial loss with his Arch-Enemy. Elizabeth spends a whole page swooning over the awesomeness of it.
Mr. Bingley, finding out that Mr. Darcy kept him from seeing Jane, mans up the independence to escape Mr. Darcy's command to go and propose to Jane. (Mr. Darcy by that time was able to bless Mr. Bingley's decision.)
Even better in the 1995 BBC version, where Bingley is clearly furious with Darcy. It's the only time in the whole story that he ever gets angry at all.
Bingley: "Then I have your blessing?"
Darcy: "Do you need my blessing?"
Bingley: "No. But I should like to know I have it all the same."
Mr. Bennet's letter to Mr. Collins, announcing Elizabeth's and Darcy's engagement: "If I were you, I'd stand by the nephew — he has more to offer."
Awesome from the Adaptations:
In the 2003 Latter-Day version of Pride and Prejudice, Darcy is the only character that remains British - with a Cool Car and accompanying guitar riff that James Bond would envy. See in the first minute here.
In the 1995 miniseries, Louisa Hurst following up Mary's frankly dreadful singing and piano with a flawless rendition of "Rondo alla Turca" at the Netherfield Ball. The moment may be just musical — and go unnoticed as most of the Bennets make spectacles of themselves — but wow. It's enough to make any musician swoon.