#1: The Manhunters

How deadly are they? They destroyed all life within Space Sector 666 because of a "programming glitch." How big is a Space Sector? Unknown, although the entire universe is split into at least 3601 of them, and Final Crisis claims that there are "thousands of worlds" within Earth's sector. So let's just say that's a lot of life extinguished.
Who's responsible? The Guardians of the Universe, the immortal blue dwarves who'd learn from the mistake of creating unstoppable killing machines and try and fix it by creating weapons of almost limitless potential and giving them to living beings... before embarrassing them by calling them Green Lanterns. As you can tell, that one worked out much better, at least commercially.
Last seen... hooking up with the fear-filled Sinestro Corps in 2006's The Sinestro Corps War, although you can probably expect to see them in this summer's Blackest Night crossover series from DC Comics at some point.

Next: See who didn't make the list.