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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I'm going to revise my previous statement and say that literally any God-Tier race in the history of Fiction. The moment they're labelled as Gods, it's basically like you're giving them unlimited power to do literally whatever the hell they want, given prep time.