Amnesiac God

The main character begins the story with Laser-Guided Amnesia and is immediately swept into strange events. Gradually it becomes apparent that the character is not a normal person like he thought but a Physical God or something very close to it. Among the things he forgot were his powers and how to activate them. When the character finally regains his full set of memories, the only thing that can stand against his Story-Breaker Power and prevent the plot from being solved in short order is another character with the same powers.

Compare Angel Unaware. A subtrope of I Am Who?. May lead to the character feeling A God I Am Not.

WARNING! There are unmarked Spoilers ahead. Beware.

Examples of Amnesiac God include:

Anime and Manga

Comic Books

This is a somewhat ambiguous example, but in the "Quantum Quest" story arc by John Ostrander in Captain Atom, Cap, who was already one of the most powerful superheroes in the DCU, discovers that he has the power to create (and ultimately destroy) his own universe. He does not, however, have the power to govern that universe at all well (hence the ultimate destruction). Of course, the story left open at least some possibility that the whole thing might have been All Just a Dream.
This is the basic plot of The Sentry, and the main thing that distinguishes him from other Superman clones. The twist is that not only does he not remember who he is, the entire rest of the world has forgotten the Sentry's existence as well.
When The Mighty Thor came back to the Marvel Universe after years of being dead, he first had to find his fellow Asgardians, all of whom, had amnesia and were under the impression that they were human.

Fan Works

Nyx. With the last revision, it's made much more explicit that Nyx is, in fact, Nightmare Moon. Thankfully the events of the story change her for the better.
Imperfect Metamorphosis. Rumia used to be a godlike, omnicidal fallen angel.

Film

John Murdoch from Dark City. He isn't the only Reality Warper, but after the Exposition Beam that teaches him how to control his ability to "tune", he becomes the single most powerful one.
The creator figure in The Nines grew obsessed with incarnating in human form and living out pseudo-normal lives. (A reference is made to Evercrack.) However, he realized that there was only so much fun in playing in literal God Mode, so he deliberately erased his own memories to make things more balanced.
An unused premise of Thor would have followed Dr. Donald Blake in his discovery that he was the reincarnated nordic God of thunder.

Literature

Nine Princes in Amber by Roger Zelazny:
- As a member of a Big Screwed-Up Family, even with amnesia, Black Sheep Corwin keeps things close to the vest.
- "Creatures of Light and Darkness", too.
The Book of Ptah by A. E. van Vogt.
Midnight at the Well of Souls by Jack L. Chalker
Lobsang and Jeremy in Terry Pratchett's Discworld novel Thief of Time.
Shadow, aka Balder in Neil Gaiman's novel American Gods.
Archer's Goon by Diana Wynne Jones
A variation occurs in David Eddings' Belgariad and Malloreon series. The child "Errand" is initially thought to be a random innocent kid, who happens to get along with the resident Empathic Weapon. As he grows up he gradually manifests weird powers, starting with surprising insight for a child, and ramping up to Nigh Invulnerability. It turns out he's really one of the world's seven Physical Gods, and simultaneously knew and was unaware of his identity. It's kind of complicated.
Hinted at but ultimately left ambiguous with Poldarn in the Scavenger trilogy (Shadow, Pattern and Memory) by K. J. Parker; part of the myth of the god Poldarn is that he loses his memory before bringing about The End of the World as We Know It, and the main character who takes on that name certainly does lose his memory, but whether he actually is the god Poldarn is never made entirely clear. Although he does bring about The End of the World as We Know It (by accident), and fulfills various elements of the prophecies of the god Poldarn without realizing it, and displays some uncanny abilities which may be superhuman or may just be down to luck and coincidence.
In Day Watch, Vitaly (the protagonist) starts as an amnesiac. He is actually a "mirror", a human warped by the Twilight to keep The Balance Between Light And Dark.
Khayman, from The Queen of the Damned, of Vampire Chronicles lore. Turns out he's more or less the third-oldest vampire in the world, not counting the King and Queen, and therefore one of the most powerful. Ever.

Live-Action TV

A sort of subversion on Angel, when Cordelia returns with no memory but when she gets them back, it turns out an ancient all-powerful evil is just using her body as a temporary vessel and they just gave it back its memory. It goes on to successfully impersonate her though and causes all kinds of hell (literally) before they figure it out.

Tabletop Games

The Celestial Exalted in the Exalted RPG are an embodiment of this. The Solar, Lunar, and Sidereal Essences are handed down throughout the millennia, but each new incarnation must relearn all the powers that its predecessors had, helped out by the fact that the Exaltation, the third soul that make an Exalt an Exalt, has some of their predecessors memories still attached. At the height of their power, they can perform feats that can put the gods to shame.

Video Games

Sonic Unleashed has Chip, also known as Light Gaia, the opposing force to Dark Gaia. While he was given amnesia due to a premature awakening alongside Dark Gaia, Sonic thought he gave to him when he landed at the beginning of the game.
Emil in Tales of Symphonia: Dawn of the New World
Gig of Soul Nomad and The World Eaters is established early (but he is initially clueless) as Death who got his ass kicked and sealed in the onyx blade. However, the exact details are a whole other story...
Kang the Mad in Jade Empire.
Breath of Fire IV's protagonist is a god split across several thousand years. He has no memories, but during the course of his journey he learns his other half saved the world and is now learning that Humans Are the Real Monsters and plans to Kill'Em All once they can merge.
Agarest Senki 2 Chaos
Tales of Legendia Grune
The main character, Vayne from Mana Khemia, kind of. Actually, he is an artificial Mana of Wishes that can cause any wish to come true, which requires god-like powers. It is uncertain whether the powers disappeared after you fight the Final Boss.
Zelda from Skyward Sword is the goddess Hylia reincarnated as a mortal. She spends about half of her time in the game getting Hylia's memories back.
The Nameless One in Planescape: Torment is essentially a Physical God, having accumulated immense knowledge, experience, and power from his past incarnations. Without his amnesia, he could legitimately be counted as one of the most powerful beings in the planes.
Neptune in Hyperdimension Neptunia is this seeing as after the Scripted Battle, she gets flung down to Planeptune with no memories of what she's done whatsoever.

Web Comics

The Everyman, the protagonist of A Beginner's Guide to the End of the Universe, is an Anthropomorphic Personification of all of humankind and basically a Physical God. He is able to manifest mild Reality Warper powers since the beginning of the story, but initially they seem just to be a part of the RPG Mechanics Verse. As he slowly recalls his true nature and sacrifices his life for the well-being of humanity, the extent of his powers grows into infinity.

Web Original

In the Avatar Adventures role-playing board, the character Sam G's powers involve manipulation of the Fourth Wall; That is to say, if his poster types something, no matter how outlandish, it comes true (for example, the user Sam G could type "all the villains die" and it'd happen). Realizing that this would make him a Boring Invincible Hero, he sealed all his powers, in addition to his memories, in a golden pocket-watch which he cast to the end of the universe.
The Epsilon AI in the The Recollections trilogy of Red vs. Blue. The final episode, coupled with Word of God, suggests that the events of the past 3 seasons were all part of a Recursive Reality created by Epsilon from his memories, who then assumed the role of his predecessor, Alpha, in this version of events. Up until now, Alpha has historically lost his memories (which are in turn reincarnated as another Epsilon) as the cycle begins anew, allowing the same chain of events to occur again. The version of Epsilon the audience sees in the final episode retains his memories this time around after restarting the cycle though, leaving the possibility of breaking the cycle.