Anti-Monitor

If Imperiex destroys planets one at a time, the Anti-Monitor destroys galaxies one at a time.

Originated from within the antimatter universe, he is much more than just a cosmic threat "“ he is a threat for the entire multiverse. With his awesome power, he easily killed Supergirl, who is believed to be even stronger than Superman. He isn't just the embodiment of anti-matter, but also the embodiment of evil.

He is the sole responsible for the Crisis on Infinite Earths "“ his fight with the Spectre resulted in the collapse of the entire multiverse. The Anti-Monitor is not at all like any of the other supervillains on this list "“ he is responsible alone for killing millions upon millions of superheroes.

Varying in size, he can be said to have to same superpowers as many other supervillains "“ but they are all enhanced, because the Anti-Monitor has fed on the energy of the millions of universe it has destroyed. So when we are saying that he can time-travel, we are actually saying that he can go to the beginning of the positive matter universe and try to prevent it from being born. If we say that he can manipulate reality, then we say that he can manipulate all the known dimensions of reality.

As far as supervillains are concerned, there are very few more powerful than him.