This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Child of the Storm
How Hela came to be. Oh Loki...
The "Mama loves you, Harry. Mama will always love you," line from a not quite dead, not quite alive Lily Potter - Word of God is being smugly silent on that particular score. It was designed to reach in and press the button marked 'Feels'.
Lily was pregnant when she died, according to Sirius. And she was planning to surprise James with the news after Halloween.
Carol'sSenseless Sacrifice in chapter 60, as they were rescued moments later. Thankfully, she's saved by the Winter Soldier, but still.
The deaths of Luna Lovegood and Arthur Weasley in Chapter 70.
Bucky's struggles with himself and his identity in chapter 71. While shaking off his programming was awesome, it's left a real mess behind.
Chapter 72: you've got Harry clinging to his godmother and weeping in her arms until he can't cry any more in response to Luna's death and his own and doing much the same with his father. It seems a very long way away from Harry's first hugging of his father 70 chapters or so beforehand, because this Cutie is very much broken.
When Carol reveals to Steve that she's his great-granddaughter, he's so shocked at the situation that he pushes her away, which leaves her hurt and confused.
Harry having to say goodbye to his mother, after just getting her back, as she has to return to her duties as the White Phoenix of the Crown.