2. The Borg
You can run, you can hide, but if the Borg decide to come after you, it's pretty much game over. A teeming collective of countless cyborg drones, all ruled by a single hivemind, they fly their ugly cube-ship wherever they please and force innocent species to join their ranks. Even Captain Jean-Luc Picard got assimilated for a brief, awful time. Can anyone who's seen 1990's classic two-part episode "The Best of Both Worlds" forget the moment when we first saw him as the creepily dead-eyed Locutus of Borg? Just face it – resistance is futile.