When it comes to Star Trek villains (perhaps even all villains ever created) the Borg reign supreme. They are like super-intelligent, super-strong, highly advanced cyborg space-zombies with a hive-like collective consciousness. They have no other purpose but to assimilate other cultures into their collective in order to gain their knowledge and technology. In the process of assimilation they destroy the target civilization.
Here is a picture of what the Borg look like.
Here is a picture of their bad ass space ship.
Maybe we should make a Star Trek thread.