Harold Jordan. In the DCU, he was the Silver Age Green Lantern and considered the greatest of the now-defunct Green Lantern Corp.
He was an Air Force pilot, then a test pilot before Abin Sur crashed on Earth dying. Sur's ring searched the planet for a person meeting the two requirements to be a GL (you must be honest and without fear). Jordan was the closest, so Abin gave him the ring.
After a distinguished career, Jordan went insane and became the entity known as Parallax, a being with near limitless power and the ability to travel through time. He tried to re-start the universe (see Zero Hour), but was thawrted by his former JLA teamates. He later sacraficed his life to re-ignite the sun during the Final Night.