3. Susan Foreman
Susan was the first Doctor Who companion. Yes, the show started when Barbara and Ian wandered inside the TARDIS and got themselves kidnapped... But Susan was there from before the beginning. The mysterious granddaughter of the Doctor, whose slight psychic abilities and amazing desire to scream at everything carried her through most situations. Susan was also the first companion to depart from Doctor Who, which makes her even more special. In The Dalek Invasion of Earth she falls in love with the sometimes cardboard-like David, and the Doctor makes an executive decision to leave her behind to live her own life. He dematerializes the TARDIS as she sadly watches him go, and that's the end of the regular appearances of Susan Foreman. The plot of a companion leaving for love has been used many times, in both better (Jo) and worse (Leela, good god Leela) ways, but the resonance of Susan's departure isn't really about that. Her farewell is so affecting because it is the very first, and so a previously undone thing. Her departure happened before anybody realized that Doctor Who would become so popular, and last so many years. It was the departure of the first companion a brave plot that laid the path for the future. Which, unfortunately, gives the impression of Susan being the first link in a chain. But, really, she's so much more than that. All companions are unique but Susan, if possible, was extra unique. She was the granddaughter of the Doctor, the only member of his direct family appearing in the TV series. This creates a certain amount of emotion that it's very hard to replicate. As the TARDIS fades away the Doctor is leaving behind his own granddaughter, and that's something that it's hard not to get emotional about. Such emotion is aided by the extraordinary performance of William Hartnell. He's the one that started it all, and arguably the one that led to Doctor Who still being around today. With Susan's departure he shows just how great he could truly be. His expression as he realizes that it's time to say goodbye is truly like a punch in the stomach. And when gives his famous speech and says "Just go forward in all your beliefs, and prove to me that I am not mistaken in mine" goosebumps are raised. William Hartnell was not just the First Doctor, but a great Doctor. His performance here makes Susan's departure perfectly heartbreaking. Susan was the first companion of Doctor Who, and the first one to leave. Her departure is truly heartbreaking, made so by both context and performance. She was one of the Doctor's most unique companions, and deserved a proper send-off. And, thanks to the performance of William Hartnell and the bravery of the decision, she most certainly got it.