It’s easy to say that the magic of Doctor Who is that it’s constantly able to reinvent itself, but that’s not entirely true. It plays a part, yes, but the real magic—the magic that has fueled the series for the last 50 years—is the relationship between the Doctor and his companion. It’s a special relationship, and without companions, the Doctor would not be the Time Lord he is today. They keep him grounded, they teach him lessons, they save his ass. And just like the Doctors themselves, fans have their own personal favorites.
David Tennant will always be my Doctor. It doesn’t mean Matt Smith didn’t worm his way into my heart over the course of the last three seasons—if I’m being honest, he did much more than that—but Tennant has always been and will always be my Doctor. But the title of favorite companion? That’s a difficult question to answer. If you’d have asked me prior to Smith’s run as the Doctor, it would have been a resounding cheer for Catherine Tate’s Donna Noble. She was adventurous, she was loud, and she was bold. She wasn’t in love with the Doctor, she was in love with the time spent with him, the times spent traveling in the TARDIS and saving the world with him. Donna was a firecracker and she was fantastic. But as Tennant left, so did Tate, and fans were, for the first time since Russell T. Davies rebooted the series in 2005, facing a new world with both a new Doctor and a new companion at the same time. Who was Matt Smith? Who was Karen Gillan? Why should we care about them?