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 don't know about the most powerful but I remember in an episode of star trek the next generation there was this immortal god like man (no not Q) living on this planet by himself (and a fake version of his dead wife.). After these bad aliens killed his real wife he lost control for just an instant and with a thought erased all 50 billion of the bad aliens out of existence. That's pretty powerful.