6. Anti-Monitor
Anti-Monitor became famous during The Crisis on Infinite Earths storyline. This is where he became one of The Green Lantern Corps' most fearsome foes. The Anti-Monitor might not be one of the most popular super villains but he definitely is one of the more powerful. He's killed Barry Allen's Flash as well as Supergirl while in the process of trying to destroy all matter in the Multiverse. What is The Anti Monitor? He was born on a moon in Qward, as a result of a civil war on Oa between the Green Lantern Corp. The Guardian of Fear is made of pure energy with powers beyond belief. He's used so sparingly that he doesn't appear in very many relevant archs because he's so powerful. His powers consist of unmeasurable strength, durability, he can fire bolts of energy, travel in time and can gain strength by absorbing universes (universes, plural). At different points through out time, he's also wielded both Black Lantern and Sinestro Corps' rings which took his powers to another level. The Anti Monitor is probably the most powerful super villain that you've never heard of.
His Weakness? His biggest weakness is his overwhelming confidence. As he demonstrated in The Crisis series, his over confidence is what ended up being the cause of his ultimate demise. He's so strong that he leans on his powers too much. In the end, it was his lack of intelligence is what led to Earth's mightiest heroes defeating him.