10. John Hurt
There remains a rather pertinent theory regarding John Hurt, whose appearance at the end of the series 7 finale has had tongues flapping and keyboards being pounded for a good few months, and it potentially fits the notion that Hurt could be The Valeyard. In "The Stolen Earth" Ten started to regenerate, but stopped it from going all the way in order to just heal himself: some have postulated that this actually makes Ten also the Eleventh doctor, and making Matt Smith the 12th and Capaldi the 13th. That would fit with the idea of The Valeyard appearing between those incarnations. That could be contradicted by the doubt over the definition of incarnation, as 10 didn't physically change when he part regenerated, making his not a true reincarnation, of course. There's also the fact that costume designer Howard Burden said that Hurt's character is a "dark Doctor" existing between the Doctor's Eighth and Ninth incarnations, which the BBC haven't confirmed, but the idea of the dark Doctor fits The Valeyard, even if the timing doesn't fit the canon. The difficulty here is that exchanges between The Doctor and Hurt don't feel like the kind of exchanges one would expect from The Doctor when faced with The Valeyard. You might expect their dynamic to be slightly different, since the Doctor has encountered The Valeyard before, but writing can always get around these issues.