We've discussed some exceptionally evil spacecraft on this list, but at least the captains at their helms are invested in what Earth has to offer: energy, princesses, sex, suffering, political power, spiritual refuge and good old-fashioned settlement territory.
Our final entry isn't interested in any of that stuff. The Vogon Constructor Fleet turns up in "The Hitchhiker's Guide to the Galaxy" not to conquer the Earth, but to bulldoze it and make way for a hyperspace highway.
The Vogons, you see, don't regard the people of Earth with any ill will -- they hardly regard them at all. As author Douglas Adams puts it, the Vogons are "not actually evil, but bad-tempered, bureaucratic, officious and callous."
All the other intergalactic nasties at least respect us enough to call us their enemy. The Vogons simply don't care, which makes their intention to destroy the Earth all the more insulting.
The enormous, bricklike vessels are as boring and destructive as the creatures who pilot them. Keep watching the skies, and hope they don't turn up anytime soon.