Hank Henshaw was/is the Cyborg Superman. He, along with 3 others, went up in an experimental rocket & were bombarded with unknown cosmic rays, and gained phenomenal powers. However, unlike the Fantastic Four, these powers were killing them -- the 'Human Torch' went insane and flew into/merged with the sun, the 'Thing' threw himself into a giant electromagnet & tore himself apart, and the 'Invisible Girl' was phasing out of reality. Hank, who seemed to be dying from rapid radiation poisoning, tried to save his wife, and Supes helped. She was eventually returned to normal, but not before Henshaw died. But he did not really die -- rather, his mind leaped into the computer console he'd been working on, and a few months later kitbashed itself a body. He went to go see his wife, but she was horrified at him. It was also discovered that his body was throwing the entire planet's EM spectrum out of whack, so he chose to leave, transferring his mind into Superman's Birthing Matrix (the ship he arrived to Earth in, which he had put into low Earth orbit for some reason). Henshaw reconfigured the Birthing Matrix into a FTL ship, and traveled the universe, slowly going insane over time. Eventually, he met Mongul (big yellow-skinned bruiser, he's been on the JL cartoon before) on Warworld. They hatched a plan to increase both their power bases, and to destroy Superman. Henshaw used some residual Kryptonian DNA from the birthing matrix, plus the technology of the Matrix & of Warworld itself, to fashion himself a cyborg Superman body. He'd learned Supes had died, and so returned to Earth, masquerading as the Man o' Steel. At one point, the Cyborg Superman took Doomsday's unconscious form and hurled it out of the galaxy, seemingly in a direction that would cause it to avoid any planets, and he placed a tracker/monitor on it so he'd know if/when Doomsday ever woke up. Later, Mongul arrived, and they both destroyed Coast City (Hal Jordan's hometown), replacing it with Warworld-esque technology -- they planned on turning Earth into another Warworld. Both were, of course, defeated.
Much later, Doomsday landed -- on Apokolips. And the tiny 'monitor' was essentially a Mindseed of Henshaw, allowing him to reconfigure the first Parademon Doomsday thrashed into a new Cyborg Superman body. Cyborg then proceeded to take over Apokolips as Darkseid scrambled his forces to defeat it, and eventually faced doomsday mano-a-mano (Doomsday actually survived a direct hit form Darkseid's Omega Beams, then proceeded to pound the snot out of Darkseid). Somehow, Superman found out Doomsday was alive & trouncing Apokolips, so he went to stop the monster. Doomsday attacked Superman almost immediately, Supes barley got away with his life. Later, Waverider (one of the Linear Men, Guardians of the Timestream) appeared and offered to help Supes by revealing Doomsday's origin.
Thousands upon thousands of years ago, on a blasted, barren planet whose atmosphere was an acidic mix of wind-blown, poisonous gasses, and whose only native inhabitant was something that makes Hellhounds look cute & cuddly, a Genetic Engineering lab had been established. The Chief Scientist (I can't remember his name, so I'll refer to him as Bob) was testing what was essentially Darwinian Evolution -- he and his assistants had taken a humanoid baby, barely a day old, and literally tossed it outside. In less than a second the atmosphere alone killed it, as Bob expected. Bob had his assistants retrieve the corpse, and examine it for any bit that, through random mutation, had been least affected by the atmosphere. They found one, cloned a new baby from it, and tossed Baby 2.0 outside. This went on several thousand times, and with each iteration Baby became tougher and stronger. Eventually, the final Baby was a large hulking grey-skinned, bone-spiked thing that not only did not need to breathe, but within a year had slaughtered every "Hellhound" on the planet.
Then Baby returned home...
Baby slaughtered the lab techs, and some time later a random cargo transport ship passed by close enough for Baby to leap up and latch on, and Baby was taken to another planet. On this other planet, Baby went about destroying everything, until the High Muckety-Mucks resorted to a drastic measure -- they turned one of their own into an Energy Being, who was able to kill 'Baby.' Per their traditions, these aliens wrapped 'Baby' in a green cloth and shot him into space, where he landed on Earth many decades later. 'Baby' then awoke, marched towards the nearest city (Metropolis), trouncing the entire JLA along the way. When Doomsday (the name given it by the media) finally met Superman, it attacked with even more rage & hatred, and in the end both Doomsday & Supes died.
But both got better. For Doomsday is Living Evolution, able to adapt to any and all threats. If something manages to kill him, he will revive, and be immune to whatever it was that killed him (the second time Doomsday arrived on the alien planet where the Elders created the Energy Being to combat him, the Elders created another Energy Being, but as it rammed into Doomsday, Doomsday's body gave off an equal & opposite energy, allowing Doomsday to tear the Energy Being into many tiny pieces). Waverider also merged with doomsday for a while, and realized that when Doomsday looked at Superman, it saw Bob -- the planet it had been created on was Proto-Krypton! For their second battle, Supes defeated Doomsday by (with Waverider's help) banishing Doomsday to the end of time, where Universal Entropy consumed him.
But then Hal Jordan, distraught over the loss of his home & all his friends & loved ones, had to get all mad & godlike, destroying the entire GL Corps and absorbing the Oan Power Battery into himself, becoming Parallax. Parallax was going to destroy & recreate the Universe per his own desires, among other things bringing back Coast City. Parallax did indeed destroy the Universe, but the other DC Heroes re-created it in a random, Big bang-like way (i.e. it was all a follow-up to the Crisis on Infinite Earths). One side-effect of this Universal Re-Writing was that Brainiac (who is a super-intelligent, super-telepathic alien mind-in-a-human's body, not a robot like in the JL cartoon) was able to retrieve Doomsday's body from the End of Time, and transferred his own mind into Doomsday's body. Eventually Doomsday rejected Brainiac's body (and his mind, ironically, was trapped in an advanced robotic body), and Supes & the JLA fought Doomsday for a third time. This time, they defeated Doomsday by tricking him into a JL Transporter, then split Doomsday among three other Teleporters, dividing his body into four partially-formed entities.
Doomsday later appeared in the Our Worlds at War mega-event, somehow brought back by President Luthor (for those who don't know, yes, in the comics, Lex Luthor is President of the USA). This time, Doomsday actually evolved an intelligence, but that did nothing against Imperiex, who disintegrated Doomsday.
I was kinda disappointed that animated Zatanna wasn't shown in her fishnets when she appeared in Batman The Animated Series.
