Sixsmith walking through the door literally seconds after his boyfriend commits suicide. Foregone Conclusion, sure, but it's the way he clutches him in his arms, the utter pain on his face, and the realization that he is soon going to discover the suicide note, whereupon he will find out that they were mere feet apart the entire time at the Scott Monument.
While many of the couples meet again in other lives, they never do.
Sonmi clutching her chest while crying, when she is told that Hae Joo is dead, as if she is feeling emotional pain unlike anything she's felt before, having been kept in such a monotonous and unengaging cycle her whole life. It was just such a broken and sad look. Luckily, Hae-Joo is alive, kicks some ass and saves Sonmi.
Then when she meets her end. She meets it with dignity, however and even while she tears up, she goes out with a smile.
The sad fate of one of Sonmi's Fabricant sisters, and the truth behind Exultation. It's bad enough seeing just how excited she is to finally retire from her awful working life; what makes it even worse is that she clearly doesn't have the slightest inkling of what's about to happen. Up until the very end, that sweet guileless look never once leaves her face. And then a slaughterhouse worker shoots her through the head with a bolt gun. As if to add insult to tragedy, the workers drop all pretences of friendliness and go about callously launching her body down the conveyer belt. Perhaps the saddest aspect of this scene is the music: by this stage, "Sonmi's Discovery" has left the loud, ominous orchestral stage behind, and continues in the form of a soft, mournful piano solo as the Fabricant's eyes go dead and the work crew dispose of her.
Imagine the facade these workers may very well be like that simply because they've grown numb with the lie. Even if they convince themselves of 'what' these fabricants are, they cannot deny the innocent, almost childlike demeanor they have. And with the knowledge to each and every one... it's enough to sap what moral joy is left in you and could certainly explain the blank, almost dead look on their faces.
Zachry's reaction when he finds the bloodied corpse of his sister, throwing himself to the ground and tearfully kissing and touching her face.
The entire ending counts as one, but in a happy tears way. Some lives end, other goes on but life always remains constant, no matter where we are. And if we are separated from the ones we love, in some way shape or form, we'll meet together again.
From the book
Frobisher visits a graveyard of soldiers hoping to lay flowers on his brother's grave, but can't find it. His letter attempts to keep up his cavalier tone, but he notes that he'd foolishly hoped he'd somehow just know which grave held him, wonders if his brother had liked men too, and remembers with resentment how their parents had said "You should be like your brother!" in increasingly poisonous ways. When the car he's in strikes a pheasant on the way back he's profoundly horrified by how it screams like a child, goes out of his way to give it a Mercy Kill, and is shaken up by the whole experience.