A bit of an odd choice for a list like this, but the Teller (from Series 8's Time Heist) is deserving of its place nonetheless. While the episode itself didn't quite add up to the sum of its parts, the alien at its heart was one of the stand outs. Firstly, it's a fantastic design. What you see on screen is actually an alien suit and the fact that a number of viewers mistook it for CGI is a credit to the costume department. Not only that, but it has a really strong concept. A creature that can feed off guilt, leaving his victims hollow shells of their former selves, is a good idea that manages to be an effective threat throughout the episode. The sense that the Doctor and Clara are more or less helpless against him only adds to how intimidating the Teller is. Not only that, but the Teller isn't actually a villain at all. At least, not of his own accord. He's forced to help the Bank of Karabraxos against his will to protect the person he loves. This adds an extra dimension to this alien, turning what could have been one-note into something more realised, making the Teller more interesting that he could have been.
