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 Ehrentil: 4/28/2015 4:23:52 PMObviously the Reaper. I mean they are unstopable without a super weapon and only one laser beam from them destroys a ship. Also they can brainwash people and learn more and effectiv tactics after each time they wipe entire species from the face of universe. They collect example of us and replace things in their weaponary to operate more effectiv and aggressiv Really impressiv