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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The Time Lords because they never age, and if you kill them they can decide what they become through the regenerative process up to 12 times (13 lives, can do it more if given more lives) and are invincible temporarily after regenerating, thusly making them practically impossible to kill.