DC: The New Gods and The Multiverse
In a Nut Shell: DC Comics mythology has always revolved around two themes, Jack Kirby’s New Gods and The Multiverse. Sure, there’s Gods, and Demons, and Angels, and etc, but even they more or less tie into either The New Gods and the Multiverse. The New Gods being a race of warring aliens, and the Multiverse being a collection of 52 alternate realities that have the habit of spilling into each other.
The Source is God. Or Creation. Or Reality. All became real with The Source came into being. It is the “source” or the New God’s power. It was actually the fighting between the New God’s that released some of that power, leading to the existence of magic and super powers. There also exists as “Source Wall” that prevents anyone from trying to reach it, and its is the ultimate gravesite of all New Gods. It, along with the New Gods, exist outside the Multiverse.
Darkseid is the leader of Apocalyse, and the literal God of Evil. His ultimate mission is to obtain the “Anti-Life” equation and conquer the multiverse. One of his more attempts led to the creation of the Justice League. Unlike most villains, Darkseid is slow and methodical, willing to play the long game for his victory.
Highfather is the leader of New Genesis. He is the main opposition to Darkseid’s ambitions. But he has his greatest warrior, and Darkseid’s son, Orion, do most of the fighting. The forces of New Genesis oppose the forces of Apocalypse.
Aside from the New Gods, the most influential force in DC mythology have been the Monitors. Originally just one being, the Monitor’s watch over the 52 worlds, ensuring that they don’t “bleed” into one another. Their action, or inaction, have led to many crises within the multiverse. They’re pretty much a less compotent version of The Watcher.
While Superman is not a cosmic level entity normally, he is central to DC mythology. In one way, shape, or form, Superman has been involved in very crisis, every major New God’s battle, and every big event. Tying it back to the New Gods, the Apocalyse and New Genesis are often called the “4th World,” and Superman can be seen as the arrival of a new mortal led “5th World.”
Marvel: The Celestials and Cosmic Beings
In a Nut Shell: While DC’s mythology is very centralized, Marvel tends to have a much broader scope of Gods and power players. And often time, most of their best stories center around ground level Earth stuff. But in terms of Mythology, much of Marvel revolves around The Celestials and Cosmic Beings. All powerful entities who see reality as more of a large experiment than anything else.
The Celestials are basically giant all powerful aliens. They created the Marvel Universe. Their origin is unknown, but they are responsible for such things are The Eternals, which Thanos is a part of, Deviants like The Skrulls, and Mutants. Their goal is to ensure that evolution continues, and if they judge that a species has reached is genetic potential, they will destroy it.
Galactus is a force of nature in the Marvel Universe. His name is feared by all planets. He is a world-eater, and the lone survivor of the last universe. A cosmic being who can be reasoned with, but rarely convinced to change his course, he is the being that Earth has had the most encounters with. In fact, it was the Fantastic Four’s defeat of Galactus that put the planet on the cosmic map.
The Infinity Gautlet and Infinity Stones
While not a comic entity, these stones and gautlet have led to many reality warping events. Each stone has the power to affect existence in some way, but together make the wearer all powerful. A few have managed to join them together, like the mad Titan Thanos, and no other comic entity have been able to match its power.
DC or Marvel?: I gatta give this one to DC. Ironically, DC has the more consistent mythology than Marvel. To be fair, however, this is a product of having built this mythology around 1971, and incorporating it around the 1985, while Marvel was adding more and more stuff since 1961. The New Gods and Multiverse are simply easier to wrap your head around and remember.