"The ultimate villain of the story, who's causing the problem the heroes must solve."
Note that Big Bad is not a catch-all trope for the biggest and ugliest villain of any given story. The Big Bad is the one who turns out to be behind several other seemingly independent threats.
- Big Finish Doctor Who has a few of its own.
- The villains for the Eighth Doctor's stories before the Divergent Universe Arc are the Neverpeople, then Rassilon. The villain for the Divergent Universe arc seem to be the Divergents, working through the Kro'ka. However by the finale it turns out the Divergents have died and Rassilon is really the Kro'ka's boss.
- The Headhunter in the "New Eighth Doctor Adventures" Series 1, though Mr Hulbert, who hired her, could be considered this.
- The Meddling Monk in the New Eighth Doctor Adventures Series 4, though at the end he is eclipsed by the Daleks, led by the Dalek Time Controller.
- Then the Dalek Time Controller becomes this for "Dark Eyes", though Kotris may hold this role.
- In "Dark Eyes 2" the Eminence, who are so dangerous the Doctor even works with the Dalek Time Controller against them. However the Master could also be considered this.