7. Clara Goes Too Far
Clara's arc saw her build herself up to believe she's on the same level as the Doctor, and acts like she's invincible. It was an interesting study of what can happen when people get too close to the Doctor if he builds them up as he did with Clara.
Of course, she also had the Impossible Girl story to back her up too. She had been the most important person in the Doctor's life with fragments of her scattered through space and time that were designed to die to keep the Doctor safe.
Clara is often criticised as a prime example of being a Mary Sue character, but under the surface there's a lot psychological damage to her. This led her to make so many brave, yet rash decisions and eventually sent her to her death. It's genuinely harrowing, how her character develops so drastically, and makes us view the Doctor, who encouraged her, in an entirely different way.
Clara's death wasn't the end though. During the events of Hell Bent, it was made clear that her Mary Sue style storyline even managed to help her escape death and fly off in her own TARDIS. Maybe it is a fitting, full-circle ending for the companion who became too much like the Doctor.