2. The Borg - Star Trek
"We are the Borg. You will be assimilated. Resistance is futile." It doesn't have quite the same catchy ring to it as
"Exterminate!" but it carries with it the same weight of threat and would pose one hell of a challenge to the Doctor. The Borg are a pseudo-race of cybernetic beings, or cyborgs, from Star Trek's Delta Quadrant. The Borg weren't truly a group of individuals (with the possible sole exception of the Borg Queen), as all Borg were linked into a hive mind and ultimately acted as a collective unit. Their ultimate goal was the attainment of 'perfection' through the forcible assimilation of diverse sentient species, technologies, and knowledge. As a result, the Borg were among the most powerful and feared powers in the galaxy, without really being a true race at all. Who better to help them achieve 'perfection' than the Doctor. Assimilating a Time Lord - and one so resourceful and with such a huge intellect - would be the final piece in their puzzle in their quest for universal domination. Whether or not the Doctor could prevent this from happening would make for one hell of a spectacle.