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 Rip Van Winkle: 3/1/2015 11:29:30 PMMe, I hold a controller and kill bowser, stomped lui kang, been mega man more times than I can count, controls empires with a mouse, defeated Zeus from my couch, bested the arbiter and laughed upon the flood, stopped nukes on the battlefield, saved Glienor more times than I can count, proved to be a better wizard than Harry Potter when I was 7, collected every gym badge and caught 'em all, defeated the mighty Rathalos and Rathian, defeated the Zerg, stopped frieze along with baby, became the strongest Jedi and abolished the Sith, and forged my own Destiny for I am Legend