Perhaps the worst of it was Corday's reaction. No screaming or wailing - just quiet, wordless, complete devastation. Bravo, Alex Kingston... bravo.
The look on Carter's face after he finishes reading Mark's letter to the staff still gives me chills, as well as his near-breakdown as he reads Corday's
Dr Burke, who successfully removed his first tumour, gives Mark the death sentence and says 'You should have been dead a year ago, Mark. You got married, you saw your daughter be born. I'd say that was time well spent'.
Worst of all is how he dies knowing full well that he can never be there for his daughters. To add insult to injury, Ella's first word is 'Dada'.
When Lucy died, because even that bastard of a doctor Romano refused to give up on her when she was clearly beyond help. The worst part is that at one point it looked hopeful, before things took a turn for the worse.
The desperate way that he reacts when she's gone, trying to go for another cycle, anything to have a chance. Corday's quiet acceptance of Lucy's death is heartbreaking, but who thought they'd see Romano trashing the room because he couldn't have a colleague? He genuinely liked her, because she was smart, gutsy, and stood up to him.
When Mark runs to Union Station to catch Susan and tell her that he loves her, only to have her reject him.
Mark's father dying at the end of season 6. At this point he has end stage Lung Cancer and is finding it very difficult to breathe. Mark gives him a sponge bath and his father tells him that he used to bathe Mark as a baby. He then says 'I love you, Mark' in the saddest way possible, to which Mark responds 'I love you too, Dad'. Mark then takes a nap and when he wakes up his father is dead.
Reese Benton being born, so weak and frail and near the edge of death.
The episode where Peter's nephew Jesse gets shot. Peter does absolutely everything he can to save the boy. Finally he starts to fall apart and says 'Come on Jesse. Come on, man. Don't give up on me. Don't give up on me'. Jesse finally dies from his injuries.
When Benton's mother dies. Throughout the entire season, we've come to know him as a very stern, stiff, gruff character who's only weak spot is his love for his mother and his complete denial about how ill and frail she's becoming. Towards the start of the episode, we've heard him give a patient's relative a very standard, "I'm sorry" speech. Then he's paged about his mother, rushes to her nursing home, only to get nearly the exact same speech from her doctor. When her physical therapist comes to comfort him, he finally breaks down and cries in her arms.
"Time of Death", with guest star Ray Liotta as a dying alcoholic ex-con. The whole episode is in real-time, and goes back and forth between his subconscious and scenes in the hospital. He ends up refusing surgery that might save his life, and Pratt (who never knew his father) takes his final request to get in touch with his estranged son.
The death of paramedic/firefighter Raul in season 2.
"Love's Labors Lost," Season 1. That is all.
Kerry telling a deaf patient about her miscarriage through sign language.
Sandy's death in season 10 was played for mass quantities of Narm, but largely saved by Laura Innes' performance. Kerry's quiet desperation as she pleads with the surgical team to let her into the OR ("You have to let me see her... that's my wife in there") is devastating, especially for a character who's spent her career being as abrasive and unlikable as possible. Especially at the very end of the OR scene, where blood begins running up Sandy's endotracheal tube(that's it in a surgical situation), Kerry's face freezes in position and she quietly tells Corday and Anspaugh: "You can stop. She's gone."
The Season 6 arc involving Weaver's beloved mentor Dr. Lawrence, who's developed Alzheimer's - specifically, his discussion with Weaver during his final appearance:
Lawrence: I saw a woman this morning - dementia. She had no idea where she was, who she was. In ten years that'll be me. Bedridden...in diapers. Locked away in some home, nobody coming to see me.
Weaver: I'll come and see you.
Lawrence: But I won't know who you are.
The ending of "The Storm, Part 2"
Luckily, it's eclipsed by the happy tears when Carol and Doug reunited.
The flashbacks to the death of Luka's family. His son is already dead when he gets there, and his daughter stops breathing. Luka spends hours performing CPR on his daughter, hoping that help will come, and only stops because he's exhausted. Then he checks his wife's pulse and discovers that she's died while he's been trying to save his daughter.
That episode makes the ending of 21 Guns even worse. As well as the Adult Fear of something happening to your pregnant partner, for all Luka knows, he's watching his wife and child die again.
Two Ships, in season 12, when two planes crash in mid-flight above Chicago.
A kindly old man Neela was talking to before she ran into a burning building was wanting to find his dog. When Neela gets back, she sees the dog beside him and happily tries to talk to him about him having found it—only to be told that the man died. Shocked, Neela can only say that she was talking to him a minute ago.
Adrian's death. His heart isn't beating, but CPR is allowing oxygen to get to his brain. They manage to keep him alive long enough for his family to get there, before he asks them to stop. It's the single tear that does it.
The season 15 episode The Book of Abby. Specifically, Haleh's hidden wall with the nameplates from departed staff.
Neela watching the video Gallant left her and finally breaking down.
The stillbirth of Carter's son. He staggers out of the delivery room, only to hear his father call his name and hold his arms out to him, at which point he collapses in tears.