The "Childhood" arc, which tells the story of how Chara and Asriel died. Standout examples from that arc include the village incident itself, Asriel's reasoning for bringing Chara's body back, Chara's last words to Asriel, and Asriel telling his therapist how these events warped his thought process and how he still blames himself for everything that happened.
The "Unpleasant" arc has Asriel's outburst to Toriel about how he killed people, and how her treatment of Asgore is hurting him. Toriel responds by coldly telling him she needs a moment and essentially shooing him away.
The "Bad" arc has Toriel and Asgore remember the day Asriel and Chara died and the subsequent declaration of war. Featuring fun moments like Asgore and Toriel realising that Chara and Asriel are gone and there's nothing they can do about it, Asgore being given the bullets that killed his children, Asgore's war declaration itself, and Toriel admitting that she also was responsible for the fallout by her inaction and that in her insistence in hurting Asgore, she was also hurting Asriel, along with her horrific realization that she failed to take either of their feelings in consideration.
It now has a comic dub, found here. Especially heart-wrenching is the way you can hear Toriel's voice breaking around 7:55 and onward, as her Jerkass Realization kicks in.
. Especially heart-wrenching is the way you can hear Toriel's voice breaking around 7:55 and onward, as her Jerkass Realization kicks in.