The Marvel Cinematic Universe has no shortage of powerful beings - but the most powerful will most likely never appear on film any time soon. From ferociously strong Earth-bound characters like the Hulk and Vision to otherworldly cosmic leaders like Thanos, Dormammu and Ego the Living Planet, powerful leaders are routinely surpassed with every film. However, the single most powerful force in the Marvel Multiverse will - for a slew of reasons - never appear in any film or TV series: the almighty and mysterious One Above All.
The One Above All is, for all intents and purposes, God of the Marvel Multiverse - and God has no equal. Consider the fact that the being known as The Living Tribunal is the cosmic judge of the Multiverse who can annihilate planets and stars in moments...and yet for all his power, even he pales in comparison to the One Above All. In 1992's Infinity War (only tangentially related to the Avenger: Infinity War film), the Living Tribunal is "...the representative of one whose might exceeds even Eternity!", referencing another cosmic being in the Universe. In the followup series Infinity Gauntlet, the Living Tribunal tells Adam Warlock "I represent forces that dwarf even your might...my authority comes from on High." It's clear that the Living Tribunal is but a servant of the One Above All - but has the One Above All ever been seen in comics?
Appearances of the One Above All are few and far between. The deity sometimes appears as a blinding white light, or a nondescript homeless man as seen in Sensational Spider-Man #40. However, one of the most notable sightings of the being was seen in Fantastic Four #511, when the team attempts to recover the soul of the Thing. Traveling through a realm that could only be described as Heaven, the four pass through a door and emerge in an ordinary living room. Before them sits the One Above All, sketching comic panels and characters on a drawing board - and he bears a striking resemblance to the legendary artist Jack Kirby.
As the co-creator of the Fantastic Four, Captain America, Spider-Man, Thor, Black Panther, the X-Men, and countless other characters over the decades, Jack Kirby is an ideal representation of the God of the Marvel Universe. Even so, he takes a phone call from a "collaborator" in a later panel (heavily hinted to be Stan Lee), much to the amazement of the Fantastic Four. Unfortunately, with the passing of both Kirby and Lee, such a scene would be difficult if not impossible to realize in the MCU. Disney would also potentially balk at the prospect of introducing a religious deity into the shared universe, and thus the One Above All is limited to the comics for the time being.
Marvel's depiction of the One Above All as a simple human artist works perfectly within their universe. As the OAA-as-Jack Kirby himself says "That's what my creations do. They find the humanity in God." The Marvel Cinematic Universe may have to go without their Creator, but the One Above All will no doubt reappear in the comics in the future.