Season 10, Episode 24: "Fear (of the Unknown)"
This might be stating the obvious, but Grey's is really about its female friendships. Derek might be the love of Meredith's life, but Cristina is her soulmate. This is the episode where Cristina Yang last graced our Thursday night screens, right before waltzing away to Switzerland to reign over a top-notch heart facility. Our beloved cardiothoracic genius says goodbye to all of the Grey Sloan surgeons and dances it out with Meredith one last time.