This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Pride and Prejudice
Mr. Bennet having to come back from London without having found Lydia, facing the awful truth that his family may very well be ruined because he refused to put any real effort into being a proper father to his younger daughters.
Elizabeth's Love Epiphany, because it comes with the conviction that they will never be together. Our knowledge of the Foregone Conclusion is no comfort to her. It's hard to see Elizabeth so shattered and depressed because she was so arch, lively and spirited before.
In Mr. Bennet's reaction to Elizabeth's engagement to Darcy, he drops the snarky mask and we get a peek of the regret he carries around with him.This line is specially sad.
Mr. Bennet: My child, let me not have the grief of seeing you unable to respect your partner in life.