The Golden AgeGreen Lantern had powers so nebulously defined as to be limitless. The Batman: Black and White story "Guardian" depicted him as essentially capable of anything he can imagine. He describes this as a curse, however, and says he quit the hero game because his power scared him, was too much for one man. In his words, Gotham needs a guardian, not a god.
In Justice League Elite, the Fourth World being Eve has the ability to reshape reality, although it appears to be limited — she can't make any changes of her own volition, and there is an upper limit to the scale on which she can make large changes (she is unable, for instance, to change reality so that Superman never existed).
Half the supporting cast of Grant Morrison's version of Doom Patrol, mainly villains and Anti Villains. A couple of heroes, too. And on the subject of Grant Morrison, the Writer from Animal Man.
The mother of Cascade in Sovereign Seven: She remodels all of her Earth (architecture, fashion, technology level, etc.) every few minutes. It turns out she began as a normal (but very powerful) superhero who prevented an alien invasion but failed to stop the aliens from destroying Earth in revenge. Then she used her powers to reanimate the world, so that the daughter she was pregnant with could have some semblance of a normal life. She's also best friends with Darkseid for some reason, but how they could ever have met was never explained.
From the Kingdom Come universe, Jonathan Kent, the son of Superman and Wonder Woman, who has full control over hypertime.
Some far future versions of Superman evolve this power including one who's the last being alive in a dying universe and holds back universe-ending entropy waves just by thinking about it.
Syrene: Do you forget Syrene's power is beyond imagination? I possess Merlin's mystic Runestone! Whatever my mind conceives — becomes frightening reality!
And from The Authority, all the Doctors, and Jenny Quantum.
Ibis the Invincible, who immigrated to DC continuity along with the Shazam! titles, has the Ibistick, a magic staff that can warp reality on command.
Legion of Super-Heroes: The Time Trapper can manipulate reality in everything BUT the present, hence his name, he traps you in time. Oh yeah, he can also move boulders.
Volthoom, the First Lantern and wielder of the first Power Ring, can manipulate time, space, and matter up to a certain area and has the potential to rewrite the entirety of reality itself.
Captain Atom became this in a story arc entitled "Quantum Quest", in which he created his own universe over which he had total control. It didn't end well. Also, it is possible that it was All Just a Dream. On the other hand, it has been hinted at that he might be that powerful in the regular universe too, but just doesn't realize it or is in denial about it. Dr. Manhattan, an Expy of Captain Atom in Watchmen, has telekinesis down to the subatomic level, allowing him to rearrange matter at will. Technically, this is a little different from changing reality outright, but since he's the most ridiculously overpowered character in the saga, nearly everyone there considers him a Reality Warper. Captain Allen Adam, an Alternate Universe version of Captain Atom from Final Crisis and The Multiversity, is similarly powerful, but being uncomfortable with his powers, takes drugs to suppress them.
Wonder Woman super-villain (though more GentlemanPhantom ThiefPost-Crisis) Angelo Bend/Angle Man has the ability to warp his surroundings, create portals through space, and walk on whatever surface he likes regardless of gravity.
Chaotic Neutral side). In Supergirl mini Supergirl: Cosmic Adventures in the 8th Grade, Mxy makes a serious bid for total omnipotence. In Emperor Joker, Mr. Mxyzptlk accidentally gives most of his power to the Joker (on the extremely Chaotic Evil side!). Earlier, at the end of the Bronze Age Batman/The Incredible Hulk crossover, the Joker got reality-warping power from the Shaper of Worlds (see Marvel Universe for more about him).