This is THE question that has plagued comic book fans for centuries. This is the question that is similar to "what came first, the chicken or the egg?" There are many different arguments, and valid opinions as to who would win this battle. Now, it is time for me to put in my two cents worth... Even if no one wants it.
This is going to work by me listing five characters, who in my opinion, would be the best in a fight to the deaths.
Known as 'The Destroyer of Worlds', Galactus was introduced to the Marvel Universe in 1966 in Fantastic Four #48. Galactus has been described as "the physical, metamorphosed embodiment of a cosmos", and he needs to devour planets that have the potential to support life in order to maintain his existence. Galactus has the power of reality alteration, energy manipulation, life-force manipulation, matter manipulation, cosmic awareness, telepathy and telekinesis. However, with all of this mighty power, he has been defeated by the Fantastic Four on numerous occasions, not to mention, he got turned into a cloud in Fantastic Four: Rise of the Silver Surfer.
Thanos arrived in the Marvel Universe in 1973, in Iron Man #55. He possess the powers of superhuman strength, superhuman stamina, superhuman durability, genius-level intellect, energy manipulation, telekinesis, teleportation and longevity. He is also accomplished at hand to hand combat, and is trained in the art of war. He has appeared in 2012's The Avengers, and 2014's Guardians of the Galaxy, and will appear in future Marvel media.
Everybody's favorite mercenary and assassin, Deadpool, was created in 1991, in The New Mutants #98. He is a mentally unstable mutant that gained his powers through participation in the Weapon X program. Deadpool is a master in martial arts, sword fighting and is an expert marksman, as well as having superhuman stamina, agility, flexibility and reflexes. But that's not all, he is immune to telepathy, and has regenerative powers, and has a device that can teleport him, and can create holographic disguises. He is set to appear in his own film in 2016, and he's going to need it after being butchered in 2009 in the abomination, X-Men Origins: Wolverine. Oh, and he has also killed the entire Marvel Universe.
4. Jean Grey/Phoenix
First introduced to comic book readers in 1963's X-Men #1, Jean Grey is an incredibly powerful mutant, and she is an Omega-Level mutant. She can use telepathy and telekinesis, and then with the help of the Phoenix Force, she can travel through space, manipulate matter and energy, create cosmic fire, resurrect herself from death, manipulate life energy, and manipulate timelines. She has been portrayed by Famke Janssen in the X-Men franchise.
5. The Hulk
First appearing The Incredible Hulk #1 in 1962, the Hulk is a key member of the Avengers, and numerous other teams, including the Illuminati. Everyone knows of how powerful the Hulk is, and he is inspired by Dr Jekyll and Mr Hyde. He has numerous abilities, including hand to hand combat, superhuman strength, speed, endurance, jumping, endurance, reflex and durability. He is also immune to disease, further powered by adrenaline, and accelerated healing, and resistance to mind control. Also, he is freaking immortal. On top of that, Bruce Banner has genius-level intellect. He has had two stand alone movies, before being a part of 2012's The Avengers.
So those are my picks for Marvel, and you'd have to agree, they're pretty formidable. Now, for DC.
First off, we have Darkseid, who is the embodiment of evil, and has even been known as Lucifer, aka Satan. He was first seen in Superman's Pal Jimmy Olsen #134, in 1970. He is the ruler of Apokolips, and is an enemy of both Superman and the Justice League. His powers includes: control of the omega force, super strength, super endurance, immortality, invulnerability, and a genius-level intellect. He has appeared once in live action, as a villain in Smallville, and has yet to be seen on the big screen.
Yet another Superman villain, Doomsday first appeared in Superman: The Man of Steel #17 in 1992. He is most well known for being the only character that has killed Superman, in The Death of Superman storyline. His powers are superhuman strength, superhuman endurance, superhuman speed, superhuman agility, healing factor, invulnerability, reactive adaption, and evolutionary camouflage. He has appeared in live action, as a villain in Smallville, and just like Darkseid, has yet to be seen on the big screen.
3. Shazam/Captain Marvel
First seen in Whiz Comics #1, in 1940, Shazam is the alter-ego of ten year old boy, Billy Batson. Batson has been given his powers by the wizard, Shazam, and they have allowed him to go toe-to-toe with the likes of Superman. His powers include superhuman strength, speed, stamina and courage, as well as near-invulnerability and flight. He also has genius-level intellect and control over magical lightning, either as a weapon or for casting spells. He has appeared in movie serials, and has had a 1970's live action television show. He is set to appear in a live action film in 2019, with Dwayne 'The Rock' Johnson cast as Black Adam.
4. Doctor Fate
Doctor Fate first appeared in 1955, in More Fun Comics #55, and there has been numerous characters in the DC Universe to take up the mantle of Doctor Fate. Compared to other characters on these lists, his powers may seem fairly limited, as he has a mastery of magic, however, this includes spellcasting, flight, telekinesis, and telepathy. Like a lot of DC characters, Doctor Fate has appeared in live action on the TV show, Smallville.
5. Doctor Manhattan
Last, but definitely not least for DC, is Doctor Manhattan. Doctor Manhattan first appeared in the 1986 graphic novel mini-series, Watchmen. He has an incredibly wide array of powers, which are: superintelligence, superhuman strength, telekinesis, teleportation, control of sub-atomic matter, force-field creation, full perception of time, physiological manipulation, immortality, ability to create life, can pass through objects, can atomically reconstruct himself, no need for food, air or water, and he can create fully independent copies of himself, and more that I haven't mentioned. He has been portrayed in live action in the 2009 film, Watchmen, directed by Zack Snyder.
While it may be close, and both Marvel and DC have incredibly powerful characters, my vote goes to DC, as they have Darkseid, Doomsday, and Doctor Manhattan, who in my opinion, is the single most powerful comic book character that has even been created. However, Marvel still has some incredibly powerful characters, and the would put up a good fight.