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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Humans. No matter what happens, they endure. Natural resistance and stubborn nature led them for over 20,000 years, continuously advancing in science. Oh, a fictional race. Ok. I would have to say The Force wielders, from Star Wars lore. They are only ever seen in the comics and crappy TV show, but they have the power to destroy the universe by thought. There is only 3 of them, and the only person who could kill them was themselves. Which they did, aided by the Chosen one, Anikan Skywalker and his mentor, Obi-Wan Kenobi. (Both are humans lol, worth mentioning Anakin was the only person who could auctually hold them off for more than a few seconds.)