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 Starfall Shadow: 7/1/2015 10:13:16 PMCelestialsapiens from Ben 10. They are [i]literally[/i] [b][u]omnipotent[/u][/b]. "Celestialsapiens are among the most powerful species in the universe, being capable of controlling reality, space and time. Their reality warping is seemingly without limit - a Celestialsapien can make its very thoughts become reality and survive anything up to (and including) the destruction of the entire universe around them. Celestialsapiens are apparently immortal, or at least extremely long-lived, as they can live for tens of thousands of years before even being considered "mature"."