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.
-
The Galactic Empire [spoiler]which was 99.9999% human[/spoiler] The Helghast Empire [spoiler]cause British space Nаzis[/spoiler] Or the demigods from LOTR [spoiler]cant exactly remember their name[/spoiler]