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.
-
I'm going to go ahead and put my tow cents in here and say the Precursors at the height of their power. And also I'm going to go ahead and break taboo by saying the Vex. AS A RACE; it is hard to top something that technically doesn't exist, and is the perpetrator of everything. Between the two it is tough for me to say.