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 know they don't warp reality in multiple universes, but the Precursors from the Halo universe. If they can block slipspace travel in a galaxy, then imagine in what other ways they can warp their universe, especially if they were at their full power. And, of course, they were the ones who created the Flood, even if the Flood is a cancerous, damaged monstrosity that the Precursors didn't expect to be created. I've heard they can also transcend the need for physical bodies.