Which empire has been the most power hungry and oppressive empire in the world?

All empires are brutal and oppressive by definition, all through history. We think of the Athenians as paragons of civilisation, but their naval empire was nasty in ways in that prefigure more modern forms of genocidal violence. In the 5th century BC, the Athenians slaughtered all the men of the island of Melos, and enslaved the women and children, simply because the Melians remained neutral in the Peloponnesian War. That was how ancient empires were made, and how the Athenians built their nice marble temples.