Imperiex
A truly fearsome foe, Imperiex first appeared in Superman (Vol 2) #153 in 2000. He is the manifestation of entropy, or deterioration, and is a form of pure energy, encased in a suit of armour that is humanoid in appearance. He seeks total perfection, and the pursuit of this drives him to destroy worlds and universes for the purpose of rebuilding.
His arrival on Earth is announced by the villainous Mongul of Warworld, who teams up with Superman to defeat the monstrosity – only to discover that the being they bested was merely a drone, and that the real Imperiex is far more vast, far more dangerous, and still approaching.
The ensuing showdown – a war that kills 8 million people – is next-level epic, as President Lex Luthor authorizes the release of the horrifying Doomsday, albeit under the mind control of telepath Manchester Black, to fight alongside the Man Of Steel. This action provides an opportunity to demonstrate the power of Imperiex, as he quickly and easily annihilates Doomsday, who had previously been known as The Ultimate Killing Machine.
For his part, Superman enlists the help of Martian Manhunter, Darkseid and Steel, and ultimately succeeds in returning the consciousness of Imperiex to the Big Bang, where he is destroyed.