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 Jeremy_The_One: 7/2/2015 12:43:37 AMThe flood. They had reached the point where they could manipulate matter at will and all life in the galaxy had to be destroyed and even then it still did not work. They also win against everything else because they use the enemy's tech and knowledge against them.