The Borg Cube
The relative terror of this entry really depends on your leanings toward collective thinking and tubes in your skull. But should this imposing vessel show up in your skies, expect automated assimilation into a hivelike world of ghastly, soulless cyborgs.
The Borg spread terror throughout the Star Trek universe, consuming whole species and massacring anyone who dared to resist their way of life. Their enormous, cubical spacecraft typically outclass anything else on the galactic highway.
In addition to a bevy of deadly energy and projectile weapons, the Borg Cube also features a cutting beam to carve up enemy ships like a Thanksgiving turkey. Its decentralized layout makes it highly resistant to a decapitation strike, and the vessel actually regenerates damaged sectors.
Resistance has never been this futile.