Everyone likes to bring up Tiny Tim, but Scrooge also forms a deep emotional bond with the Ghost of Christmas Present and his death at midnight of December 25th is an emotional turning point in many versions.
There's a scene in the book where Bob Crachit goes up to Tim's bed, and his body is still there, and he kisses his face. Some film adaptations also show Tim's body.
In the 2009 animated adaptation, Scrooge seeing Bob mourning Tiny Tim's death in the future is even worse than usual, because while going up the stairs to sob alone, Bob pauses for a long moment right in front of the invisible Scrooge, giving both him and the audience a long look at his utterly heartbroken face. Scrooge whimpering "Bob..." as he sees how hurt he is makes an already brutal scene even harder to watch.
In the 1951 film Scrooge, as Scrooge goes back to his childhood in the deserted school, his sister comes in and he forgets and tries to embrace her. Alastair Sim was brilliant. That one terrible, overjoyed cry — "Fan!" and she runs right through him. Oh my God.
And then there's her death, which is shown in this movie unlike pretty much any other adaptation and the book. As she's dying, younger Scrooge is overcome with grief and leaves the room. Immediately after, Fran says her final words, asking Scrooge to take care of her son, and dies. Older Scrooge's reaction makes it even worse.
Also, there's the alternate future where Tim dies. A weary Bob Crachit tries to put on a brave face for his family, talking about how Tim is at peace now. Finally, though, he just breaks down in tears for his dead son.
And there's a brief but effective scene as Fezziwig, driven out of business, watches as his sign is taken down from the warehouse. It gets worse when Scrooge, while sympathetic, just can't bring himself to go talk to the man he helped to bring down.
That scene was brutal and really shows how Scrooge was becoming Scrooge
When Scrooge sees his name on the gravestone and finds out it was his death they were celebrating. Both Alastair Sim and George C Scott are excellent in this scene, but George C Scott really brought it, especially when he fell at the the Ghost of Christmas Yet to Come's feet sobbing and pleading "Spare me!", then appears back on the bed.
This one needs mentioning—the Mr. Magoo adaptation, and young Ebenezer singing "I'm All Alone in the World".
The scene with the destitute couple on the street
I see Tiny Tim and raise you Jacob Marley. Especially in the 1984 film adaptation. He sincerely regrets never being as charitable as he ought to have been in life, and honestly wants nothing more than for his old friend to avoid making the same mistake, so it can be difficult to keep a straight face while watching that scene.
In one musical theater adaptation of A Christmas Carol, the story is narrated by a young gentleman who walks with a limp, who retells the story that Scrooge himself told him years ago. During the scene with Christmas Yet To Come, there's the expected tear jerker scene with the Crachit family mourning Tiny Tim's death. They sing the appropriately-tear jerking "The Little Child" until they are too choked up to continue, at which point the song is finished by a verse sung by the narrator, which is the only time the narrator sings outside the opening and closing musical numbers. This verse becomes extra chilling in hindsight when, at the end of the story, it is revealed that the narrator is Tiny Tim, all grown up and healthy thanks to Scrooge. Not only is Scrooge in-story witnessing the possible future of the death of a poor child, but also out-of-story the narrator is watching his own family's reaction to his death in an alternate timeline and, much like Scrooge, he cannot comfort them. Chilling.
In any version, when Scrooge's fiancee leaves him after seeing values money more than he values her. Especially when old Scrooge is shouting at his younger self to go after her.