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 anakinasvader: 5/27/2015 2:27:15 AMTimelord here's why First of all they can travel through space and time to wherever they want to. In some cases travel through alternate dimensions and can potentialy genocide the universe if they felt like it. Also they are kind of immortal.