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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Precursors. Time Lords. Forerunners. Vex (Vault of Glass variants as they can delete you by rewriting what happened earlier). ...Any other indestructible, OP species? Maybe Reapers (Reaper Indoctrination).