I've been rewatching old Star Trek episodes and I have to say the Borg hold up very well as villains. There's something so creepy about hearing hundreds of them speaking in unison:

"Freedom is irrelevant
Self determination is irrelevant
Your archaic culture is authority driven
It will be adapted to service us
You have been chosen to speak for the Collective
You will be assimilated
Resistance is futile."

It still gives me the shivers even twenty years after it was created.

Who's your favorite villain of television or film?