8. Hayley Atwell
There certainly isn't a formula to what makes a successful Doctor Who companion -- what works for one Doctor will be painfully out of step with another. But some of the most memorable sidekicks have been the ones who were a little bit more mature when they began their adventures with the Doctor -- Ian, Barbara, Donna Noble. And with the Twelfth Doctor's personality taken into account, it seems like he'd appreciate someone a little less wet behind the ears than say Rose Tyler or Jo Grant. To that end, someone like Hayley Atwell would make a perfect fit. She's not "mature" by any means, but she's far from being a bright-eyed ingenue like a lot of other companions. She's a skilled dramatic actor, and we can only imagine the delight of her and Peter Capaldi sharing the screen together. She projects a sense of confidence and unflappability, and has a way of seeming strong and forceful without becoming a tough girl stereotype. A character played by her would be a lot of fun for writers to work with.