4. Light
Light always looks really interesting to me, because while hes clearly evil, hes not a typical megalomaniac like other Doctor Who villains. His motivation is strange, and thats what makes him unique. Simply put, hes an observer, someone who wants to catalogue all life in the universe. Its what this led to that makes for all the drama and plot. Light wanted to record all life in the universe, but what with it being so vast and changeable realised that he would never be able to fully complete his catalogue. So in his infinite wisdom, he decided that he would try and wipe out all life so he could actually complete the damn thing. Again, I think thats really interesting the idea of seeing someone so fascinated in all of life would provide an interesting counterpoint to the Doctor, and give him the chance to evoke some disgust (and lord knows, Capaldi can do good disgust) when he figures out what Lights grand plan actually is. My only hope is that beating him isnt as easy as it was for the Seventh Doctor, who merely told him that he himself was evolving, so he couldnt even catalogue himself. He took that as an invitation to blow himself up, robbing us of a proper conclusion.