1. The Master, A Renegade Time Lord
The Master has been Moriarty to the Doctor's Holmes ever since the character was created. Indeed, that was his initial thumbnail sketch when he was introduced against Pertwee's Third Doctor. In those days he was played by Roger Delgado, who played him as a fairly suave, slightly dangerous, and occasionally well-mannered. Where the Third Doctor was often arrogant and rude, and certainly very condescending, the Master was calm, polite, and would take the time to explain what he was doing and why. He'd kill you, of course, but he'd never be rude about it. All that changed when the Master showed up in the Fifth Doctor's era. Beginning with the last two Fourth Doctor stories, he was played by Anthony Ainley, who did his best with what he was given. Sadly, he became essentially a mustache-twirling bad-guy in the Snidely Whiplash mode. His plans became increasingly absurd and the character lost almost any sense of credibility as a villain. He recovered slightly in the last Seventh Doctor story, but all that was sidelined when Eric Roberts played a version of the character who was so campy, even Lord Baden-Powell would have suggested he should decamp. The character continued his slide into sideshow freak status when he showed up as John Sims; a version that, somehow, came back to life using a horcrux and wound up with Force lightning. Bringing this character back to something menacing is difficult, but not impossible.
Who Should Play Him?
Well, we already know the answer, don't we? Benedict Cumberbatch, the man with the name so Dickensian that even Charles Dickens would have said, "That's a bit much." Cumberbatch would be the perfect choice, and I suspect he could play well off of Capaldi's Doctor. He could bring a real air of threat back to the character, much as he did when he played John "Definitely not Khan, why would you think otherwise" Harrison in Star Trek. Cumberbatch proved in that movie that he can play the polite, menacing sort of villain that the Master should be. He should be a "plans within plans" type of character who can be charming one moment and terrifying the next. Cumberbatch has the skills to play that better than just about any major actor working today. Plus given his history working with Steven Moffat, I think his appearance on the show at one point or another is likely. Might as well have his appearance be playing the character he was born to play. All of this, of course, assumes that Gallifrey, and the Time Lords, will be returning. I think it's reasonably likely, and I hope that they do. As I said they are, for better or for worse, a major part of Doctor Who history and they should be back on the show.