Amazo

Amazo is arguably one of the most powerful supervillains to ever face the Justice League (in the DC animated universe) - although technically speaking Amazo was not "evil", he was kind of without morality in the traditional sense of the word and although he began his life as an energy-absorbing android by his second encounter with the League he had amazed godlike power and gained a new look, his once silvery skin now replaced with golden armor.

Amazo originally absorbed the powers of the entire core Justice League members during his battles with them shortly after his creation but decided to explore space rather than continue fighting - during his travels across space Amazo amassed unfathomable power and decided to return to Earth to seek out his creator, Lex Luthor.

In the process Amazo apparently destroyed the world of Oa because it was in his way - prompting the Green Lantern Corps to pursue him, in reality however he had simply shifted the planet into another dimension and returned it to its rightful place with a simple thought later on.

Amazo defeated all of Earth's heroes with ease and even shrugged off Luthor's attempt to reprogram him, having managed to completely change his structure from its original nanotech - in short Amazo was all-but-omnipotent, however Amazo eventually revealed his true reasoning behind returning and it was not malicious: he merely wished to know what his purpose was and although Luthor mocked him he ultimately became redeemed as a student of the mystic known as Dr. Fate.