Despite already being one of the most single powerful beings in existence, Superman has actually gotten some pretty impressive power boosts. This includes his golden-form from the far future. But Final Crisis revealed perhaps the most abstract power boost in the entirety of the DC Universe, and it involves a Dragon Ball-style fusion of two distinct Supermen.
What is the Thought Robot, the ultimate and metaphysical version of Superman -- and how did it take a Doctor Manhattan stand-in to achieve that level of power?
WHAT IS THE THOUGHT ROBOT?
The Thought Robot was created by one of the earliest established protectors of the then-developing DC Multiverse, the first Monitor. Affected by the growing DC Multiverse and uncertain of how it would continue to develop over time, the original Monitor departed the Multiverse - leading to the birth of the Monitor and Anti-Monitor who would later appear in Crisis on Infinite Earths. The Monitor eventually created more versions of itself to populate the multiverse and help him watch over creation - eventually leading to the birth of the Monitor Dax Novu.
Fearing that an eventual evil could someday appear in the universal story of the multiverse that the Monitors wouldn't be powerful enough to counter, Novu constructed the Thought Robot - a being powered by the inherent potential of story. Eventually transforming into the evil he had once feared, Dax Novu's Thought Robot remained hidden and lost until the events of Final Crisis: Superman Beyond. Teaming up with a group of other versions of Superman from across the multiverse (including the Captain Marvel of Earth-5, Overman of Earth-10, Captain Atom of Earth-4, and Ultraman of the Anti-Matter Universe) and the Monitor Zillo Valla to save all of existence, the group ventures into Limbo and is almost doomed.
The Doctor Manhattan-inspired Captain Atom realizes that the heroes need to fully confront the metaphysical threat of the Dark Monitor with similar power. Using his own reality-altering abilities, he's able to fuse Superman and Ultraman - complete opposites who'd threatened to destroy reality if they even touched - into a single pure Superman. This turned them and their story into the necessary energy to fuel the Thought Robot, which is revealed to resemble a giant mechanical Superman. Superman's greater willpower allowed him to gain control of the machine and was able to use it to fully confront a corrupt deity like the Dark Monitor on its own scale.
The Thought Robot was designed to always be triumphant and defeat evil, on a purely metaphysical level. It is beyond the scope of space and time. It can survive the universe destroying attacks and has limitless stamina and endurance. No matter what, the Thought Robot - in the form of Superman - will always save the day. It's perhaps the most powerful form of Superman to ever exist, even potentially dwarfing the Golden Superman in terms of raw power. It exists on a reality-bending level of power that out-strips almost all characters. Only other medium bending characters can even begin to operate on the same level as the Thought Robot.
It gives Superman the power of the story itself, allowing himself to see the plot happening around him as that - plot. He sees through the lines of reality. and can even alter it to help good triumph. The Thought Robot can't be broken, because at its heart the story of Superman can't be broken. It's the inherent optimism of the DC Universe, made real. All of this gives Superman a serious power boost and more than enough energy to confront the Dark Monitor and help defeat him.