The one defining trait of Joker is that he never tells anyone the truth about his origins, and when he does, he often tells a conflicting story to someone else in short order. The movie world has tried to define his origin story, with 1989's Batman making him the man who murdered Bruce Wayne's parents and 2019's Joker claiming he was a failed comedian.
In the comics, there have been many times that Joker's origin played out, and almost every time, DC walked back on it later. Even when it seemed the Joker origin story was definitive, DC left wiggle room, claiming it was out of canon or misdirection in one way or another. However, DC has tried to explain Joker's origin many times.
The Killing Joke
The Killing Joke is one of the most controversial and well-known Joker origin stories. In this series, Joker is almost like he was in the Joker movie in 2019.
He was a comedian with a wife, and he wanted to support their family, especially after she ended up pregnant. While the woman in the movie was not really with Joker, in this case, she was. When he decided to pull off a heist as Red Hood, his wife was murdered, and he went into the heist and fell in the acid while battling Batman.
Three Jokers
The most recent attempt to tell the origin story of Joker came in Three Jokers by Geoff Johns and Jason Fabok in 2020. This was a fascinating story that supposes there has been more than one Joker throughout the years, explaining why he often changes character. There is the Criminal from the classic Batman era, the Comedian that shot Barbara Gordon, and The Clown.
In this series, they wanted to create a fourth out of the man who killed Bruce Wayne's parents but found Batman, Batgirl, and Red Hood opposing them. The Comedian here was the same character from The Killing Joke, and Batman reveals what really happened to the man's wife.
The Man Behind The Red Hood
One of the oldest Joker origin stories was "The Man Behind The Red Hood," which took place in the pages of Batman and Detective Comics. Batman is teaching a class in criminology at Gotham University, and one of the studies he shares with the students was a case he had involving the mysterious Red Hood.
When the Red Hood shows up at the school, it turns out to be a gardener who said he kidnapped the original Red Hood and used the identity himself to commit crimes. Then Joker showed up and admitted he was the original and explained how his fall occurred, starting with his job as a lab worker.
Case Study
Batman's "Case Study" was a 1996 story by Paul Dini and Alex Ross (Kingdom Come). In this series, which contains Ross's lifelike artwork, Joker was a crazed gangster who created a new persona known as Red Hood to commit petty crimes.
This version of Joker is very different than almost any portrayal of the madman. This is because this Joker is completely sane and only pretends to be insane to avoid getting the death penalty for his crimes.
Lovers & Madmen
"Batman: Lovers & Madmen" is a 2007 series by Michael Green and Denys Cowan that played out in Batman Confidential #7-12. This story's timeline was early in Batman's career and tried to show what Joker was like before becoming who readers know him as.
Much like in 1989's Batman movie, Joker is Jack, a young man who works as a hitman and becomes obsessed with Batman, doing everything he can to draw him into a game of cat and mouse.
Small Problems
"Small Problems" is a very different Batman origin story because it has almost nothing to do with Batman and is an Atom story. This took place in The Brave and the Bold Vol. 3 #31 and has Atom shrink down and head into Joker's mind to try to cure him of a rare disease.
There is one problem. While in his mind, Atom sees Joker's memories from his life before he was the Joker, including how he killed his parents and murdered animals before he finally became a criminal who would even kill his own accomplices.
Batman: Endgame
"Batman: Endgame" was a six-issue storyline played out in 2014's Batman #35-40. Joker had disappeared in the "Death of the Family" storyline, which was his first time back in DC Comics. Scott Snyder and Greg Capullo served as the creative team.
The story opens with the Justice League all attacking Batman, and once he has them subdued, he asks who was responsible, and it was Joker. In this story, Joker is revealed to be an immortal who has lived in Gotham forever, and it ends with what could have been the final confrontation between Batman and his greatest foe.
The House Of Hush
"The House of Hush" was a 2010 storyline by Paul Dini and Dustin Nguyen and has Hush forced to live as Bruce Wayne, who had been missing for a year.
After an Arkham Asylum breakout, the real Batman shows back up and starts to investigate. During this, Little Italy tells Hush a story of when he was starting and had an encounter with Joker, who was also new in his career as a villain. His story was horrific and showed that Joker was pure evil from the start.
Batman: Zero Year
In 2013, Batman: Zero Year came out, and as the title suggests, this was the first year of Batman's career as a crime fighter. This series was meant to give a definitive new origin for Batman in the New 52, which also meant retelling the origin stories of some of his main villains.
There was no Joker, but there was a Red Hood, which shows his first battles with Batman, leading to him eventually jumping into a vat of chemicals.
Batman #1
The first time that Joker ever appeared was in the first Batman comic book. Batman had been around for about a year before this, appearing in Detective Comics, but the regular Batman series introduced real villains, including Joker and Catwoman in the first issue.
What is important about this issue was that Joker had no origin story, he had no real name, he had nothing but the fact he was a face-painted bad guy who Batman couldn't catch. Joker's best origin was that he has no true origin and will always be an enigma.