For all the so-called rivalry between these two comic worlds (and I say so-called, because there isn't one), there does seem to be a lot of crossover/similarities between the two. My question is which came first and who has copied the other more often? Let's take a closer look at some famously similar characters.
Elongated Man and Mister Fantastic
Similarities: super stretchy, super geniuses, both married to gals named Sue.
After a long time interest in "india-rubber men," Ralph Dibny distilled the raw essence of the gingo fruit, a rare fruit from the Yucatán to which most people are severely allergic. He found that after ingesting his home-made concoction, he had the powers to stretch his body, though he did have to continue drinking the gingo extract every few days to maintain his power.
Reed Richards is perhaps the greatest living genius in the Marvel world, proficient in almost all fields of science and technology. After a successful launch into space with the four friends, intense cosmic rays penetrated the ship's shield, irradiating the crew which led to a mutation that turned them into super humans. With Reed gaining stretching powers, he became known as Mr. Fantastic and the leader of his mutated quartet, The Fantastic Four.
Contrary to what you might think, Elongating Man actually predates Mr. Fantastic over a year, with Ralph Dibny's first appearance in The Flash #112 of May 1960 and Reed Richard's later in The Fantastic Four #1 of November 1961.
Copy Score: Marvel-1, DC-0
Catwoman and Black Cat
Similarities: cat-themed-thieves wearing latex who often become romantically involved with their frienemies.
Selina Kyle has served as a morally ambiguous character, perhaps even more of an anti-hero, finding herself in the role of adversary, crime fighter, and love interest (with Batman). After years of training, her stealth and agility make her a world-class criminal, and her expertise of martial arts make Catwoman a formidable opponent in any scenario.
Walking in the footsteps of her father, Felicia Hardy became New York's premier cat burglar. Now reformed, she continues her walk on the wild side as an adventure addict and detective in the form of Black Cat.
The cat themed burglar first appeared in the form of Catwoman in Batman #1 in Spring 1940. Black Cat wasn't seen until The Amazing Spider-Man #194 in July 1979, so despite her added use of "hexes" in her (eventual) powers, Black Cat is still just a similar character brought to life nearly 40 years later.
Copy Score: Marvel-2, DC-0
Green Arrow and Hawkeye
Similarities: really really good at shooting arrows, often having trick arrows.
Billionaire Oliver Queen is the vigilante known as Green Arrow who fights crime using archery, technology, and martial arts. Frequently found with Black Canary, long-time love interest, and Speedy, his side-kick; this extended family of heroes which work with him call themselves "Team Arrow."
Orphaned at an early age, Clint Barton was sent to a children's home before he and his brother ran away to join the Carson Carnival of Traveling Wonders. Trained in the art of archery, he decided to become a costumed crime-fighter, after witnessing Iron Man save the lives of some people at the carnival.
Unsurprisingly, Green Arrow was seen first in the pages of More Fun Comics #73 in November 1941, while Hawkeye didn't reach comics until Tales of Suspense #57 in Spetember 1964.
Copy Score: Marvel-3, DC-0
Darkseid and Thanos
Similarities: humongous, powerful, destroyers of worlds.
If hatred were personified, you would get Darkseid; a ruthless dictator who demanded a steadfast and fearful dedication from all his subjects. Though he is unparalleled in strength, Darkseid prefers not to enter into combat unless provoked. His eyes emit ray beams that could disintegrate, teleport, or resurrect (Omega Sanction). His mission in life is to find and gain control over the Anti-Life Equation in order to give him the means to take possession of all free will in the universe.
The last survivor of the original settlement of Eternals on Titan, Thanos eventually became obsessed with the personification of death. In order to make himself worthy of such an awesome entity, Thanos decided to acquire more power through augmentation of his Eternal strengths and powers, and the conquest of worlds. Eventually he learned of the Soul Gems, which were six powerful objects of unknown origin and vast power, and he set about a search for them so that he could bring about the annihilation of everything that lives.
While Thanos was the first to make it to the big screen (in the Marvel Cinematic Universe) Darkseid made it to comics first in Forever People #1 in February 1971.
Thanos wouldn't make it to the pages of Marvel comics until 2 years later in Iron Man #55 in February 1973.
Copy Score: Marvel-4, DC-0... Not looking too good Marvel.
Deathstroke and Deadpool
Similarities: both mercenaries with a regenerative healing factor and a more violent tendency than most.
Known in the DC world as the greatest assassin/mercenary, Slade Wilson is primarily an enemy of the Teen Titans. Formally a soldier in the Army, he too became part of an experimental super-soldier project in which he gained enhanced strength, agility and intelligence. Deadpool's name, "Wade Wilson," is a joke suggesting that he's "related" to "Deathstroke/Slade Wilson."
Wade Wilson became a test subject in the joint U.S ./Canadian superhuman enhancement project, the Weapon X, in order to cure him of his cancer. Not unlike the recent movie, his story is one of violence, love, comedy, revenge, and witty one-liners. What make Deadpool stand out from other heroes is his ability to break the forth wall, knowing that he is a comic book character, which also allows him to do things that other characters cannot.
With the nod to Deathstroke in Deadpool's name and appearence, it's obvious that the first character to appear would be Deathstroke, in New Teen Titans #2 in December 1980. How much longer was Deadpool's first appearance? New Mutants #98 came out 11 year later in February 1991.
Copy Score: Marvel-5, DC-0
Aquaman and Namor
Similarities: conceited, self-confident, not well liked, rulers of underwater kingdoms.
The telepathic ruler of Atlantis, Aquaman is an Atlantean with incredible strength and speed as well as the ability to command all sea-life. One of the founding members of the Justice League, his particular physiology allows him to survive on land and at the ocean's most extreme depths of pressure and temperature.
Namor the Sub-Mariner was the first Marvel character with a history that spans over 70 years of publication and the only character on Marvel's roster to have the unique hybrid of Atlantean and mutant physiologies (said to be an Alpha-Level Mutant).
Namor was the first official Marvel superhero, first appearing in Motion Picture Funnies Weekly in April 1939, making him the original in this similarity battle.
Despite being more widely known, Aquaman appeared later in More Fun Comics #73 in November 1941.
Copy Score: Marvel-5, DC-1
The Flash and Quicksilver
Similarities: quick of wit and quick of foot.
As protector of Central City and Keystone City, The Flash fights evil using his super-speed. With a long legacy, the Flash Family spans throughout history using the enigmatic Speed Force to gain their powers. In the Golden Age, Jay Garrick was the original Flash and a founding member of the Justice Society, while the Flash of the Silver Age was Barry Allen who was a founding member of the Justice League.
With a strange history of being genetically altered as a child by the High Evolutionary and disguised as a mutant (depowered, repowered by Terrigenesis, later restored by unknown means), the son of Magneto and sister to Scarlet Witch, Pietro Maximoff has spent time as both a villain and a hero in the Brotherhood of Mutants, the Avengers, and the Inhumans in his lifetime.
The first appearance of The Flash was Flash Comics #1 in January 1940
First appearance of Quicksilver? The X-Men #4 in March 1964
Copy Score: Marvel-6, DC-1
Bumblebee and Wasp
Similarities: these women can both shrink to size of an insect, have the ability to fly, and they essentially have the same name...
Scientist Karen Beecher, girlfriend of Teen Titans member Mal Duncan (the Herald), secretly made herself a bumblebee-themed super-suit and attacked the Teen Titans in order to help make the Herald look good in front of the team. Later, after explaining what she had done, the Teen Titans were impressed enough to offer her membership.
In order to bring her father's killer to justice, with the help Henry Pym, Janet van Dyne underwent a biochemical process (involving his Pym Particles) allowing her to shrink to insect size. Pym also set her up with wrist devices that allowed her to fire blasts of compressed air that she called "wasp's stings" along with bio-synthetic wings for flight. Falling in love while pursuing their crime-fighting, Pym and van Dyne went on to become founding members of the Avengers.
Bumblebee's first appearance was in Teen Titans #45 in December 1976 while the Wasp made her first appearance nearly 13 years earlier in Tales to Astonish #44 in June 1963.
Copy Score: Marvel-6, DC-2
The Atom and Ant-man
Similarities: geniuses, physicists, inventors, that can shrink down to an incredibly tiny size.
The Atom is a legacy super-hero name, primarily associated with the ability to shrink in size. The original was Al Pratt during the Golden Age, but many have taken the mantle since including Ray Palmer, Adam Cray, and Ryan Choi (until his death). Palmer is perhaps the most well known, formally being a member of the Justice League, Teen Titans, Suicide Squad and Indigo Tribe.
The Ant-Man suit, created by Dr. Henry Pym, utilizes subatomic particles dubbed "Pym Particles" which reduces the wearer to the size of an insect. Pym also succeeded in creating a "cybernetic helmet" which would enable him to communicate with ants through transmitting and receiving psionic/electrical waves. Though Pym has taken on many other identities, the Ant-man pseudonym has been taken by others, with Scott Lang currently in the role (in films and comics).
Despite the first appearance of The Atom in Showcase #34 in October 1961, Ant-man's similarity seem to be more widely recognized, being released a mere four months later in Tales to Astonish #27 in January 1962.
Copy Score: Marvel-7, DC-2
Guardian and Captain America
Similarities: fighting for truth, justice, and the American way with the use of their shields.
The Guardian is a superhero legacy that is notorious for its many clones. The (original) Guardian, Jim Harper, was cloned by Project Cadmus, and several other clones were made from the genetic material of either Harper or his clone. Other people who took the alias were Mal Duncan and Jake Jordan.
Captain America was created during the early 1940's as part of a covert US military experiment turning Steve Rogers into America's first Super-Soldier. In the closing months of World War II, Captain America was presumed dead and a few decades later was found trapped in ice and revived in the modern world.
Another close call, being released only a month from each other, Captain America wins on originality with his first appearance in Captain America Comics #1 in March 1941and the Guardian's first appearance in Star-Spangled Comics #7 in April 1942.
Copy Score: Marvel-7, DC-3
Galactus and Imperiex
Similarities: wielding the power of the universe, they are destroyers of worlds and galaxies.
Galactus, the sole survivor of a universe that existed prior to Marvel's current Multiverse, originally was a humanoid named Galan. In his current form, now one of the most powerful creatures in the Marvel universe, Galactus' survival depends on absorbing the Life Force from planets or stars.
Imperiex, the embodiment of entropy, has repeatedly destroyed the universe to create a new one from the ashes of the old since the dawn of time. Detecting imperfections in the fabric of the universe, he eventually came to destroy Earth, the planet which held the universe together. Before arriving to Earth, Imperiex had demolished countless other planets.
Galactus has a rich history in the Marvel universe, first appearing in Fantastic Four #48 in March 1966. The more recently created Imperiex first appeared in Superman vol. 2 #153 in February 2000.
Copy Score: Marvel-7, DC-4
Cyborg and Deathlok
Similarities: durable, super-humanly strong with mechanized body parts and cybernetic enhancements.
Half man and half machine, Victor Stone is the superhero also known as Cyborg. After his body was destroyed in a tragic accident, he was saved through an experimental technology that took his body parts are replaced them with a large arsenal of gadgets, weaponry, and the provision of life support. Cyborg has been a primarily member of the Teen Titans, but he is also a member of the Justice League of America.
Originally a computer programmer working for Roxxon Oil, Michael Collins thought that his programming for artificial limbs would be used to help handicapped individuals. To his bewilderment, it was actually being used for the Deathlok Project, a project putting people's brains into cyborg bodies. After the discovery of his work being used to create killing machines, his boss had his brain placed into the Deathlok cyborg body.
Despite being more popularly know, Cyborg's first appearance in DC Comics Presents #26 in October 1980 was six years after Deathlok's first appearance: Astonishing Tales #25 in August 1974.
Copy Score: Marvel-7, DC-5
Now this is clearly not all of the copies, nor a definitive "who has copied the most," but a fun thing I thought I'd look into.