This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Mary Russell
This troper actually teared up when Russell rescues Holmes from those Muslims in "O Jerusalem." Russell has looked up to Holmes who has been practically raising her since she was 15 years old. He gets kidnapped and it takes days for Russell to find him, all the while thinking that he's probably dead. When they do find Holmes, he's tied up by his hands to the ceiling and his feet barely touching the floor. His back is covered in blood, for he has been beaten every single day since he's been kidnapped. He can't even speak because he's in so much pain. To see the intelligent man that Russell looked up to beaten beyond all belief, so much so that he has terrifying nightmares for weeks after he's rescued; it would be very difficult for anyone to witness that.
She can't even look when a Buddhist priest wants to examine the wounds on his back. She can't accept that this amazing man could have been almost beaten to death.
When Russell first tells Holmes about the death of her family in The Beekeeper's Apprentice.
In the second book, the moment I'm thinking of can also be considered as a Crowning Moment of Heartwarming as this is the moment when Russell comes to acceptance with her love towards Holmes but the events which transpires to get to that point is tearjerking to say the least. She gets kidnapped and becomes a prisoner in a basement where her only contact with other people for around fifteen days is her kidnappers who only show up to inject her with heroin. In her drugged state she imagines Holmes being there and guiding her through survival. Holmes' reaction to this is also heartbreaking considering his physical state when they do find her.
No matter how matter-of-fact Russell tries to be about telling it, her story of how her family died is pretty heartbreaking. Fortunately for her, she got thrown out the back window, but she had to watch the car go off the cliff with her mother screaming.