Monarch. Originally, he was an oppressive tyrant (and a former hero gone bad) from the year 2030 who had murdered all superheroes. Originally, he was supposed to be Nathaniel Adam, aka Captain Atom, but word leaked before the revelation and DC scrambled to change his identity, so he ended up being Hank Hall, aka Hawk, of Hawk & Dove fame. It was a foolish notion, as Hawk battled Monarch face-to-face in a version of his future where he destroyed Monarch. But then, of course, Waverider really only saw a variation of Hank Hall's future, not his actual one, so I guess it still worked, although there were other reasons why Hank Hall made a poor choice.
In any case, Hank Hall went mad after the future Monarch murdered Dove, so in response he murdered Monarch...and then became Monarch for the first time. Time travel, right? Anyway, Monarch went on to be a villain, although actually he didn't really get to do very much as one. He battled Captain Atom at various points in time during the ARMAGEDDON: THE ALIEN AGENDA mini-series, after which he was promptly lost in time somewhere. He later returned and became Extant, which is the guise he used to cause the most trouble. He was also killed under this alias, also.
Unlike other characters who slipped through the cracks of each Crisis event and survived as new iterations, Monarch continued on with other characters taking on his identity, causing multi-dimensional havoc whenever he could. After ZERO HOUR, a second Monarch showed up in the pages of EXTREME JUSTICE. This one turned out to be Nathaniel Adam, the man who Captain Atom was supposed to be. It turned out that, during the experiment in the 1960s that made him Captain Atom, Nathaniel became trapped in the quantum field where the excess alien metal from the experiment formed a replica of Nathaniel,after which it re-entered the timestream in the mid 80s as Captain Atom, unaware it wasn't the original Nathaniel Adam. Meanwhile, still floating in the quantum field, the real Nathaniel Adam encountered Monarch, who had found himself trapped here after the events of ARMAGEDDON: THE ALIEN AGENDA. Nathaniel. trained Hank Hall to manipulate the quantum field and gain powers of time travel. Hall escaped back into the timestream and later became Extant, but he before he did that he sent the Monarch armor back into the quantum field for Nathaniel Adam to use. Adam became Monarch and traveled to the 1990s. HE began a plot to control the world through genetic implants he inserted into people, including a dying Booster Gold, under the guise of healing them. But Booster's future technology resisted the implants, and ultimately Monarch was defeated.
And then along came INFINITE CRISIS. History was changed, and Monarch was no more. However, Captain Atom was still around, and his alien shell body was breached by one of the Monitors, who was playing a deadly game of cosmic chess with Darkseid. With dangerous radiation leaking out of him, the Atomic Knights placed Captain Atom inside an updated version of the Monarch armor. Captain Atom went nuts, murdered Major Force, and obliterated the rest of Bludhaven. His mental faculties continued to deteriorate as he entered the Bleed, the space between multiverses, and traveled around to expand his knowledge of said newly discovered multiverse.
After he accumulated said knowledge, Monarch gathered a colossal army of soldiers from across the entire multiverse in order to destroy the Monitors. He planned to destroy the multiverse and condense it to just one universe, one only he would rule. Soldiers in his army included the Crime Society of Earth-3, the JLAxis of Earth-10, Forerunner of Earth-48, the Extremists of Earth-8, and Red Son Superman of Earth-30. He chose other soldiers who battled for the right to be in his army in the pages of COUNTDOWN ARENA. Eventually, however, Monarch encountered Superboy-Prime, and the two fought until Prime ripped Monarch's chestplate open, creating an explosion that devastated the entire Earth-51 universe. An amnesiac Captain Atom resurfaced sometime later back on Earth, and it is assumed the Monarch armor was destroyed for good.
And then came FLASHPOINT. So far, Monarch has yet to reappear. Captain Atom has, in a new iteration, but he is a force for good. However, some things should be noted. During a storyline in THE FURY OF FIRESTORM, THE NUCLEAR MEN, Captain Atom was forced to absorb a massive amount of energy that split his molecules apart into three pieces that were thrust into the timestream. One such piece appeared in the 31st Century as Nathaniel Adym, an agent of Echo, a covert branch of the Science Police assigned to monitor the timestream. A mission took Adym to the 21st Century where he encountered members of the 31st Century Legion of Super-Heroes who had become trapped in the era. A singularity bomb exploded during a battle that launched Adym further in to the past, where his mind became twisted and he took on the persona of Harvest, channeling deadly energy through a staff he created. He founded the organization N.O.W.H.E.R.E. to experiment on young metahumans for undisclosed purposes.
A second piece of Captain Atom remained in the present day, until an explosion caused by his own powers hurled him back into the 80s in the pages of THE FALL & RISE OF CAPTAIN ATOM. He returned to the present, but five years later, from 2012 to 2017, causing him to be MIA during those missing years.
The third piece of Captain Atom has yet to appear. In an alternate future, Captain Atom exiled himself to Mars to avoid harming humanity and slowly went mad from isolation. This version could be the third missing piece, but seems unlikely. In any case, considering how this variant Captain Atom went mad, and how the Nathaniel Adym piece also became twisted, we maybe seeing some version of Monarch appear at some point in the future.