Season three, episode five
Our relationship with Brianna as a character is hit or miss, but when she tells her mother she has to travel back in time to try to find Jamie, their subsequent goodbye is heart-rending. Sophie Skelton and Caitriona Balfe both hit it out of the park.
Bonus scene: this episode also features Frank's death, and Balfe also nails Claire's grief — grief for a man she did once love, grief for the past 20 years of her life without Jamie, grief for her daughter, who just lost the only father she's ever known. This is an all-around great episode for Balfe.