The endgame. You break out of a millennia-old tomb/monolith, completing a brutal timed puzzle while experiencing some of the most horrific Nightmare Fuel the series has ever brought to bear.
The Whisperer, a Mayan scribe, who told the truth about her king knowing that he'd kill her for doing so.
Danger by Design
The final battle against the culprit. Yes, I said battle.
Pretty much everything Noisette Tornade did was this. As Nancy puts it, "Her flaw wasn't doing too little for her country, but trying to do too much."
Ghost of Thornton Hall
After you finish the game, Fridge Brilliance turns Nancy's name on the gravestone into this. Think about it. The graveyard is the only part of the property that wasn't destroyed, and in future years, Nancy will be remembered amongst the Thorntons, living and dead, as someone who respected them and brought them peace. The carving is no longer a source of Nightmare Fuel, but a sign of triumph and hope.
Jessalyn's actions pre-game. In the middle of preparing for her wedding, a ragged, paranoid woman who every Thornton thinks is utterly crazy manages to get Jessalyn away from the others and essentially tells her, "Your mother killed my sister, and I think I can prove it- but I need your help." And Jessalyn stays calm enough to A,) Listen to Harper and acknowledge that she may have a point, and, B,) Go against years of lies and enforced tradition to bring her own Evil Matriarch to justice.
Colton, once he realizes the extent to which he's been manipulated.
Nancy: What about your parents?
Colton: They'll have to start caring about my opinion. Because I am done with caring about theirs.
Harper solidly establishes herself as a Determinator once Nancy learns what she's really doing. Harper saw Clara lit the match that led to Charlotte's death, but no one believed her when she tried to tell them that. Depending on your interpretation of the characters, Harper may have been committed to an insane asylum because she wouldn't back down. By the time the game starts, she's been ostracized from her own family for years, and yet she never doubts what she saw. She never stops trying to reveal the truth and bring Clara to justice. For all her humourless jokes about being crazy, Harper is very sane, and confident enough to know it.