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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Mass Effect Krogan Sure they are ugly, but awesome looking! Before the genophage, they could reproduce very quickly and were bred for war. They have powerful regeneration abilities. They are highly resistant to bio-hazardous conditions. They are fun to talk to. They find enjoyment in blasting monkeys with high powered weaponry. They head-butt like a BAWS! They are stubbornly loyal. Give them a shotgun, and they will hold a position forever.