5. Mogo
He may not socialize, but as the biggest and most powerful of any Lantern who ever existed, Mogo played one of the biggest roles in the Corps, a spiritual compass that sent each Lantern ring to the appropriate bearer. Mogo also provided a spiritual sanctuary to broken Corpsmen who needed help in the worst way. As a sentient planet, Mogo was capable of great feats of power (as scores of Black Lanterns learned the hard way) but it was the emotional core that kept the Lanterns sane. Mogo's destruction at the hands of fellow Lantern Jon Stewart sent shockwaves not just through the Green Lanterns but through the readers as well. How could the Corps survive without Mogo guiding the rings? The loss is still felt today but there remains hope that it will return and provide the shining (green) light for ring slingers to follow once more.