This includes fantasy and sci-fi.
My choice is the Combine.
A multi-universal species from half-life so absolutely immense and powerful that they reverse-engineered earth's technology, upgraded far beyond human ability, brainwashed some humans and had them invade themselves. They did this and stomped all resistance within 7 hours. This was done by a backwater lesser combine city mind you.
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Edited by Jono: 2/22/2015 9:04:37 AMThe Protoss - Don't need food/water to survive. - Can't be infected by parasitic life forms. - A single Protoss warrior can take out squadrons of enemies. Injured Protoss can still fight in Dragoons and Immortals. Some Protoss warriors can move as fast as lightning when attacking or moving. - Can warp in troops/ships/buildings anywhere across the galaxy almost instantly. - The Protoss can construct intelligent machines to fight beside them if Protoss numbers are too low. The Protoss Colossus, Reaver, and Sentry are some of the machines they can create. Unknown number of those machines are hidden away in worlds, waiting to be awakened when needed. - The Dark Archon, the most feared Protoss entity, can use Mind Control on its foes and will forever be under the Dark Archon's control. - The Protoss Mothership, the biggest and most powerful ship in the Protoss fleet, can cloak entire fleets of ships. It can also shoot out black holes and suck anything unlucky enough in its radius. They also have other powerful ships such as the Carrier, Tempest, and Arbiter. - Protoss can even time travel, but they don't do it because they don't see the need for it. But in this case, they can go back in time and make sure your race never existed.