While not a single DC cosmic character, the Green Lantern Corps is the peacekeeping force that rules over the entire universe. There are a total of 7,202 members of the Corps with two covering each of the 3,600 sectors of the universe -- with the exception of sector 2814, which includes Earth.
These are not only some of the most powerful characters in the universe, but they are a military unit that grows in power when they work together. They have one of the most powerful weapons in existence with their power rings and each Lantern was chosen based on their indomitable willpower and character, only the best of the best.
Hal Jordan was seen as one of the greatest superheroes of all-time and the most powerful member of the Green Lantern Corps. He was a man who knew no fear and always did what was right. However, Parallax is an alien being that feeds on fear and was able to attack Jordan when he was at his most desperate.
Parallax was so powerful that he drove the noble and fearless Hal Jordan crazy, in turn making the Green Lantern into the universe's deadliest villain. He used Jordan to destroy the Green Lantern Corps and the Guardians, turning him into a villain that Earth's heroes couldn't even stop. Only Jordan's purity of heart was able to turn him around and only the ultimate sacrifice beat Parallax.