6) Clara Oswald
Jenna Coleman’s Clara Oswald has the honour of being the longest serving companion of the modern series. While most stayed for one or two seasons, Clara travelled with both the Eleventh and Twelfth Doctors across three seasons (that aired over four years).
And there’s a good reason for that. It’s hard to deny that the Impossible Girl is the most important woman in the Doctor’s life. At one point, Clara was fractured across the time vortex, which meant she was able to watch over the Doctor throughout his timeline. Later, she gave him the inspiration to save his home planet instead of destroy it. Last but not least, she also encountered the Time Lord when he was just a child, and potentially gave him the inspiration to be the hero he would later become.
Even death couldn’t stop Clara, and she found a way to become immortal and travel the universe in her own TARDIS. Given the character’s unstoppability, then, it’s no surprise that she’s set to return to the show for a cameo in Peter Capaldi’s last episode this Christmas.