Doctor Who: 10 Villains The Doctor Couldn't Defeat
7. The Valeyard (The Ultimate Foe)
In the series-long story Trial of a Time Lord, the Sixth Doctor's greatest foe is the legal system, with the character being put on trial by the High Council of Time Lords.
But a more specific thorn in his side is the Valeyard, who represents the prosecution counsel. The Valeyard really wants to ruin the Doctor's day - at one point, he suggests that the death penalty should be used as a punishment, because he's a dick - and this massive hate-boner persists throughout the entirety of the series, culminating in a face-to-face final showdown between the two characters.
This results in the Valeyard being presumed dead, after he's hit with an energy surge during the final moments of the serial The Ultimate Foe. The Doctor assumes all is well, and flies off in the TARDIS, ready for a fresh adventure. But then, at the very end of the story, the Valeyard is shown to be alive and well, laughing into the camera.
He hasn't returned in a televised story since then, but he's out there somewhere. And let's face it, he's probably up to no good.