Source: dc.wikia.com

2. Parallax

BIG YELLOW SPACE BUG!!!!!!!  Possessing yellow space bug!!!

So I actually like Parallax as an idea. I know, I'm the only one.

Here's how Parallax came about. In the late nineties, DC decided to get rid of pretty much all of the Green Lanterns in favor of just one brand new one. So in The Death of Superman, one of the minor things that happens is that Mongul and the Cyborg Superman destroy all of Coast City and kill everyone in it while Hal Jordan is away. This (plus the fact that The Guardians won't let him fix everything, because "rules") drives him totally nutterbutters coo-coo for cocoa puffs insane. He freaks out, and murders a bunch of Green Lanterns and Guardians, leaving newbie Kyle Rayner as the only one remaining.

But years later, they wanted to bring Hal back and make him a hero instead of a mass murderer. So the retcon fairy waved her wand, and all of a sudden, it turned out that Hal wasn't evil; he was just possessed by an evil space bug named Parallax who was the embodiment of all fear. Parallax went on to possess numerous other people, including Kyle Rayner himself, as well as The Flash and Sinestro. Weird evil space bug indeed.