Well, if you ask me it is difficult to, even subjectively, decide. As there are so many ways in which an empire could be 'evil', ranging from cruelty, intolerance to hypocrisy.
So I divided them into categories:
Most people killed: Mongols
Worst for their time: The CSA or the Ottoman empire, for seceding to preserve slavery/selling rebellious Greeks into slavery in the 19th century.
Worst colonizer: my own country thanks to that bastard of a Leopold II, or maybe the Spanish Encomienda system, though in the last one part of the blame can be laid on colonial elites instead of the central government.
Worst treatment of conquered people: The Assyrians?
Worst treatment of own people: Zhang Xianzhong's Daxi dynasty
Most hypocrite: the USA, claiming everyone was created equal but holding slaves.
Most religious intolerant: Spain, I guess. Timur or the Delhi sultanate might have been worse but there it overlapped with 'worst treatment of conquered people'.
Mmh, I'm actually surprised the Aztecs didn't end up there somewhere.