If you guessed Lady Catherine, then I can only assume it's because you've read the book before because otherwise you'd be about as surprised as Mrs Bennet and Kitty (who were less surprised, we are told, than Elizabeth - have you noticed, by the way, that Mary seems to have been almost entirely done away with recently? But I digress).
Lady Catherine in the house
Lady Catherine turns up and comes inside - in that, at least, she's better than her daughter. She leaves her attendant in the carriage. She has come from Rosings, in Kent - a fact we learn when she says she saw the Collinses two days ago. She has spent quite a long time travelling to pay what turns out to be a very rude visit (on both sides, if I'm being hones, although truly, Lady Catherine has it coming to her).
She enters the house and sits down without a word. As the highest ranking female in the room, she is entitled to precedence of a sort, even if she is a guest, but truly, she should have attended to introductions first and waited to be asked to sit. Of course, shortly after sitting down, she criticizes the grounds (the "park") as small, then criticizes the sitting room as "inconvenient" because it faces west, and might therefore be expected to be hot and/or sunny in the summer. And then, she cuts Mrs Bennet off (which is, on the one hand, a sort of small mercy, because Mrs Bennet does rattle on, but still - she's being terribly rude to her putative hostess), stands up, and pretty much demands that Lizzy take a hike with her. Lady Catherine is then rude in a different manner by opening doors to rooms inside the house to inspect them; at least she pronounces them to be okay. Yeesh.
Lady Catherine in the prettyish kind of little wilderness
As they're walking along in silence, Elizabeth cannot believe that she ever believed Mr Darcy was high and mighty in the way that his aunt was. It's a nice introduction to the topic of Darcy, although Elizabeth and Lady Catherine are about to get into quite a discussion about him. On entering the wilderness, Lady Catherine truly veers into a wild course of action, pretty much attacking Lizzy, who is completely clueless as to why Lady Catherine is in her yard.
Turns out that Lady C has heard a rumor that Darcy intends to marry Lizzy, and she's decided to meddle in her nephew's love life. She has apparently decided that, as between the two of them, Lizzy is the weaker link, so she's started her assault at Longbourne. She's hoping to be told that there's no change of an engagement between them, but since she starts out so rudely, Elizabeth immediately gets her back up and starts toying with Lady Catherine over semantics, more or less.
Lady Catherine: You know why I'm here, Miss Bennet.
Tell me what I want to know.
Indeed, Madam, I have no idea what you're doing here.
Lady Catherine: A report reached me saying you're going to marry my nephew, something I consider impossible. I am positive it's a pack of lies, which is why I've just spent two days in a carriage to hightail it here.
If you think it's impossible, I don't know why you bothered coming all this way.
Lady Catherine: Well, obviously, I want you to confirm that it's all a pack of lies.
But doesn't your coming here seem to prove it might be true?
Lady Catherine: Look, I'm sure it's you who's been spreading this rumor. I want you to tell me there's absolutely no basis for it whatsoever.
I don't actually have to tell you what you want to hear.
Lady Catherine: *footstomp*
HAS MY NEPHEW ASKED YOU TO MARRY HIM?
"Your ladyship has declared it to be impossible."
Lady Catherine: *mumbles* Well, it should be impossible, unless he's a cottonheaded ninnymuggins. But you may have
If I had done such a thing, I'd hardly admit it.
Lady Catherine: "Miss Bennet, do you know who I am? I have not been accustomed to such language as this. I am almost the nearest relation he has in the world, and am entitled to know all his dearest concerns." [KRF: LOL! I'm betting Darcy wouldn't see it quite the same way!]
Be that as it may, you have no entitlement to know my business.
Lady Catherine: Darcy is going to marry my daughter! They're engaged! SO THERE!
If that's the case, you can't have any reason to think he'd ask me to marry him.
Lady Catherine: Did I say engaged? I meant that my sister (his mother) and I planned on them getting engaged. You know - keep it all in the family. The wealth and the genes. So clearly, propriety demands that you bugger off!
Elizabeth: Even if I were to bugger off, it wouldn't guarantee that he's going to marry your sickly daughter. I'm just saying that you obviously failed the course on logic. So if he asks me, why couldn't I accept him?
Lady Catherine: O_o
Look, it's like this: his family will never talk to you or even say your name, and the whole world will censure you.
Elizabeth: Is that seriously all you've got? Because I'm betting there are enough perks to being Mrs Darcy that I wouldn't mind the family bit, and I seriously doubt that the entire world would give a rat's ass.
Lady Catherine: "Obstinate, headstrong girl! I am ashamed of you!" *sits down to belabor the many ways in which Elizabeth is a disappointment, including mention of "The upstart pretensions of a young woman without family, connections, or fortune. Is this to be endured! But it must not, shall not be. If you were sensible of your own good, you would not wish to quit the 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."
Lady Catherine: "True. You are a gentleman's daughter. But who was your mother? Who are your uncles and aunts? Do not imagine me ignorant of their condition."
Elizabeth: "Whatever my connections may be, if your nephew does not object to them, they can be nothing to you."
Lady Catherine: "Tell me once for all, are you engaged to him?"
Elizabeth: *wishes she could say she was*
"I am not."
Lady Catherine: Promise me that you won't get engaged to him.
Lady Catherine: Come off it. You can't marry him. Besides the excellent reasons I've already given you, there's your youngest sister's patched-up elopement - and with the son of Mr Darcy's steward! "Heaven and earth! -- of what are you thinking? Are the shades of Pemberley to be thus polluted?"
Elizabeth: O_o! "You can now have nothing farther to say. You have insulted me in every possible method. I must beg to return to the house."
Lady Catherine: *heaps scorn and invective on Lizzy, then asks if she's determined on having Darcy - seriously, if she wasn't determined before, this sort of effort might have been enough to get her to commit just to piss Lady Catherine off*
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."
Lady Catherine: "I take no leave of you, Miss Bennet. I send no compliments to your mother. You deserve no such attention. I am most seriously displeased."
The 1995 BBC adaptation did a terrific job with this, although they did cut some of the dialogue (as did I, for that matter). By all means, stop at the 5:37 mark unless you wish to peek ahead into the next chapter: