There are so many wonderful moments in Pride and Prejudice that it's hard to limit it to one "part." I think the most compelling scene for me as a teacher is the moment when Elizabeth is visiting with her aunt and uncle at Pemberley on their tour in chapter 43. She stands at the window and looks out on the grounds and says:
"Elizabeth . . . went to a window to enjoy its prospect. The hill, crowned with wood, from which they had descended, receiving increased abruptness from the distance, was a beautiful object. Every disposition of the ground was good; and she looked on the whole scene -- the river, the trees scattered on its banks, and the winding of the valley, as far as she could trace it -- with delight."
Although this is an odd choice for a favorite passage, I think what Elizabeth is reflecting on is not Pemberley at all, but Darcy. He's "beautiful" with a "disposition" that was "good." Everything she now knows about him she sees "with delight." What a change of heart! Austen is so subtle here---making Pemberley stand for the man. It's clear that Elizabeth has changed her mind about Darcy, and Austen makes it clear to us that she has through this description. I think Elizabeth is looking at the "prospect" of Darcy in a new light. She is indeed seeing things with new perspective out a clearer window. Clever Austen. Beautiful change of heart.