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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Unicron from Transformers. Damn near unkillable (seriously, he's been decapitated, blown to pieces, had his spark extinguished, etc. and it's only slowed him down), the Transformers equivalent of Satan, unbelievably ancient, "blood" is capable of raising an undead army of Transformers, turns into a planetoid that literally eats other planets, has an insatiable hunger, despises any and all living things, etc.