Season 1: Katie Bryce
Katie Bryce (Skyler Shaye) gets nostalgia bonus points for being Meredith's very first patient. Her burst aneurysm puts the entire show in motion. It's the first case that Derek and Meredith ever work on together, and we get literal chills when she pops up again in season 12 after Derek has passed away. Plus, Katie's case builds the foundation of Meredith and Cristina's friendship. Cristina comes to understand why Derek lets Meredith in on the surgery and keeps her secret about McDreamy.