The-One-Above-All (Also referred to as Above-All-Others) is the supreme being of Marvel Comics, as well as the superior of the cosmic judge known as The Living Tribunal. Having very few mentions and even fewer appearances, The-One-Above-All has very rarely interfered directly in the events of any Marvel comic, only ever doing so with a conversation with Peter Parker, reviving Ben Grimm and restoring Universe-616 after it had been destroyed.
Marvel's first mention of the existence of a Supreme God, in Fantastic Four #72
Eternity explains that neither it nor it's sibling Death are God, for while they encompass the universe, God rules all universes, in Doctor Strange #13
Thor talks about the Multiverse's cosmic hierarchy, and how God must be immensely higher than all his creations combined, in Thor Annual #14
First comicbook mention of The Living Tribunal being merely the servant of an even higher being, in Fantastic Four Annual #23
The Living Tribunal explains that his authority "Comes from on high.", in Warlock and the Infinity Watch #1
First comicbook usage of the title "The One Above All", in The Infinity War #2
The Protege attempted to gain absolute power and become the new One Above All, in Guardians of the Galaxy #50
The Fantastic Four's encounter with God in the form of Jack Kirby, in Fantastic Four #511
Odin mentions the existence of a supreme God, in Thor Annual #2000
The first physical appearance of The One Above All, talking with Peter Parker in The Sensational Spider-Man #40
Mephisto explains that The Living Tribunal isn't God, but merely "The biggest kid in all the playgrounds. And if he knows the Principal, he's not exactly chatty about it.", in Journey into Mystery #627
Becca Steinhardt prayed to the "Creator of all Gods and Men", saving her's and Thor's lives from the deity known as Glory, in Chaos War - Thor #2
Thanos and Adam Warlock meet with The Above-All-Others in a realm beyond all time and space, in Thanos: The Infinity Finale
The One Above All makes a personal statement in the Ultimates issue #100
What is likely The One Above All takes the guise of a bald parrot and confronts Dimitrios in a void beyond reality, letting him know that 10,000 years in the past, his plan has already failed.