6. Gus (Mummy On The Orient Express)
The Great Intelligence isnt the only disembodied entity that has menaced the Doctor since Moffat has been in charge. Theres also Gus, the computer from Jamie Mathiesons brilliant Mummy on the Orient Express, who turned out to be secretly in control of anything. Gus is polite and courteous - and initially seems anything but sinister - but that just compounds his villainy when it comes to light. Despite being deferential, theres no doubt that he is in charge of the Orient Express and that the many experts have no choice but to do his evil bidding. Hes also utterly ruthless - hes perfectly willing to destroy carriages of innocent lives to get his way, as he does just to get the Doctor to end a phone call. Hes perfectly willing to kill to cover his tracks, too, with only the Doctors quick actions saving lives when Gus drains the oxygen from the Orient Express and blows up the train for good measure, having got what he wanted. Theres still a lot of mystery about Gus, too. Hes been in the shadows for a while, being the mysterious caller to the TARDIS as far back as The Big Bang, and his awareness of that number and of the Doctor means theres probably more going on here. Theres also the question of just what he wanted the Foretolds secrets for. All in all, while Gus has already been an effective threat, theres a lot of potential for more from him in the future.