Despite being one of the most dangerous men on Earth, the Joker is outclassed by plenty of cosmic threats from across the DC Universe. This includes Darkseid, who is usually held up as a nearly all-powerful villain. However, that doesn't mean there haven't been occasions where the Joker has figured out how to defeat figures like that -- especially with a power boost at his disposal.
Although the Joker normally can't contend with Darkseid, the events of "Emperor Joker" gave the Clown Prince of Crime a solid win over the Lord of Apokolips.
Emperor Joker
Mister Mxyzptlk is one of the most powerful beings in all of the DC multiverse. The near-omnipotent, 5th Dimension prankster, who is capable of rewriting reality on a whim, has a perchance for jokes. One such gag ended up almost dooming the entire universe in the "Emperor Joker" storyline.
While traveling the world, Mxyplitk found the Joker locked away in Arkham. Somewhat intrigued by the Clown Prince of Crime, Mxyzptlk decided to give him 1 percent of his power, which allowed the Joker to have a major advantage the next time he came up against Batman.
The Joker was crafty enough to trick Mxyzptlk, instead figuring out how to steal the other 99 percent of his power and leave Mxyzptlk with almost none. Joker quickly remade the Earth in his image, killing his enemies and corrupting the heroes of Earth into comical figures. As his power grew, the potential for the villain to destroy everything became more and more likely. To stop this, a band of cosmic figures rallied together to try and find a way to stop him -- but all that did was open them up to a specific (and bizarre) attack from the Joker.
How Did Joker Defeat Darkseid?
A meeting of the Quintessence -- an alliance of some of the universe's most powerful heroic figures, including Ganthet, the wizard Shazam, Zeus, Phantom Stranger and Highfather -- was called in deep space. The threat of the Joker was so great that even Darkseid met with the group.
Although he didn't respect them, he was aware of their power and of the threat posed by the Joker in his current form. He even discovered the Joker had figured out the Anti-Life Equation, enraging Darkseid and leading him to try and put aside his hatred for Highfather long enough to speak with him.
Knowing full-well what could happen if the Joker were to fully unleash the potential of the Equation, Darkseid recommended attacking Joker. He asked the Quintessence not to interfere with his work and allow him to save the universe by defeating Joker. Unfortunately, the group has already been targeted by the Joker and reduced to parodies of themselves.
A massive green light appeared before Darkseid, revealed to be the Joker. The Joker teased him over his failure to take the Anti-Life Equation before Joker could and used his powers to overwhelm Darkseid's mind. He leaved Darkseid deep in space after considering making him a "hood ornament," quoting the same '90s Bud Light commercial that the rest of the Quintessence had been forced to repeat.
Darkseid isn't the only cosmic figure to fall to the Joker, but it's a nice reminder the villain isn't all-powerful. Only one cosmic figure is shown to have the willpower to resist the influence of the Joker: The Spectre, who at this point in time was connected to the spirit of Hal Jordan.
Despite his reluctance and even an attempt to combat the influence of the Joker, a genuine reality-warper is beyond even the Lord of Apokolips. Darkseid doesn't appear in any other major role throughout "Emperor Joker." It seems that even Darkseid has the potential to be driven mad if confronted by the Anti-Life Equation himself.