The Joker is notoriously Batman's greatest enemy, but can also often be considered his other half. The Joker seems to go back and forth between wanting the Dark Knight to be defeated and wanting to endlessly go through this cycle of cat and mouse that they always seem to be playing. Understanding that the Dark Knight doesn't want to kill, for sake of breaking his moral code, but always trying to push him to do so.
This being considered, the Joker has actually saved Batman on more than one occasion. If he isn't the one to put the Bat over the edge, then no one can. Joker is often considered to be the cruelest villain, evil for the sake of being evil and enjoying every moment of the chaos, but there are rare occasions when he does "good" in his own twisted way, even if it means he can just keep his game of cat and mouse going.
10 The War Of Jokes And Riddles
The War of Jokes and Riddles was a storyline in which the Joker and the Riddler go head-to-head to duke it out. All of the villains from the Batman universe chose sides and end up going to an all-out war in the streets of Gotham, totally destroying the city and its people.
Batman, of course, tries to stop this war but realizes he cannot do so alone. He joins the side of the Riddler, hoping that his side can put a stop to the Joker. But the Riddler has other plans. He pushes Batman's buttons a bit too far, causing the Dark Knight to try and stab the Riddler in the face. But Joker stops him just in time. He throws his hand up in front of the blade, stopping Batman from ruining his own code of conduct, stopping him from killing.
9 Arkham Knight
The Arkham games feature what many fans consider to be Joker's prime. Mark Hamill returns to voice the Clown Prince, and gives fans the greatest depiction of the character, as his obsession and love for Batman is never-ending. In Arkham Knight, Joker lingers as a figment in the mind of Batman.
He pops up every so often to give the Bat his opinion of different things, and sometimes even gives Batman a few clues. At the very beginning of the game, Batman is awoken in an already exploding ACE Chemicals by the Joker illusion. While this is just Batman being affected by Scarecrow toxins, it is also like the memory of Joker has saved him and kept him going.
8 Helped Him Fight The Batman Who Laughs
In the comic series, The Batman Who Laughs, Bruce Wayne from Earth-22 is pretty much exactly like the Bruce Wayne of the core universe. But this Batman had a bit of a twist. In a fit of rage, he kills the Joker and is then infected by a Joker Venom that releases from his body postmortem.
Batman of this world ends up being totally corrupted and kills all of his allies, and totally destroys his world before invading Prime Earth. Throughout this series, the Joker helps Batman in fighting the Batman Who Laughs and saves his life on occasion.
7 Stops Batman From Killing
After a ton of back and forth between the Batman Who Laughs and Prime earth Batman, the latter decides he needs to see the world in the former's eyes to really understand how to defeat him. Batman goes a little crazy after he injects himself with the same toxin and starts beating Laughs to a bloody pulp, cursing and thanking him for giving him this relief that he has needed for so long.
Alfred is begging his Bruce to stop before he does something he regrets. Joker appears and aims his gun right at Bruce, and simply says "you're welcome" before firing a non-lethal shot, stopping Batman and giving Alfred enough time to extract the venom. Joker saves Batman here from being something he isn't: A killer.
6 White Knight
Batman: The White Night sets fans up in another alternate universe where Joker has his alter-ego, Jack Napier, from the 1989 Time Burton film. After one particularly brutal fight with Batman, the Clown Prince is dosed with pills the temporarily cure his mental health.
The Joker, now Jack Napier, manages to become a normal citizen after leaving Arkham Asylum and makes a note that all Batman and the Asylum do is reinforce mental illness with brutality rather than actually helping. He becomes an esteemed citizen (while still doing some bad on the side) but ends up saving the entirety of Gotham City, including Batman.
5 LEGO Batman Movie
The LEGO Batman Movie pokes a lot of fun at the strange dynamic between Batman and Joker. Often sounding like they are in a lover's quarrel, Batman tells the Joker that he isn't is arch-nemesis, that he doesn't actually hate him or care about him.
This breaks the Clown's heart, leading Joker to assemble an anti-Batman army filled with all of the Bat's adversaries. After Gotham City begins to crack in half, Batman asks Joker and his army for his help, by telling him that he was just putting on a front, that Joker is his number one enemy, and that he really does hate him. This cheers the Joker right up, and he agrees to help save Gotham and Batman.
4 Alexander Luthor
Alexander Luthor is a pretty bad guy. He is the son of Lex Luthor and Louis Lane from Earth-3 and is actually the sole survivor. After traveling to Earth-Prime, he does some pretty messed up things, like kills Super Boy and seriously injures Nightwing.
In a fit of rage, Batman almost destroys Alexander, with the drive to kill him, but is stopped by Wonder Woman who had let her emotions get to her the same way, killing someone prior. Batman steps down, while the Joker steps up. He appears near the end of the series, upset that he hasn't gotten to "play" in the last year of events because of all of the things Alexander had done. He knew of Batman's almost breaking point and decides to finish the job so the Bat wouldn't have to.
3 The Justice League Of Arkham
This was a very short-lived dream, as this unlikely band of enemies gathered together to try and save Gotham. While the Joker doesn't necessarily save Batman in this story arc, he does inadvertently help save all of Gotham City. A new supervillain threatens to release a toxin upon the water of Gotham City that will kill every form of life that comes into contact with it.
Batman, Catwoman, and Nightwing break Joker, Poison Ivy, the Ventriloquist, and Scarface, and the Riddler out of prison and enlist their help in stopping the man. Everything goes swimmingly until the Joker decides to take a moment of opportunity and escape the Dark Knight. Nightwing and Batman leave to chase him down, leaving the rest of the team to watch the supervillain who ends up escaping to San Francisco. While the bad guy gets away, he still ends up leaving Gotham and its water alone.
2 Would Go Sane Without Him
Without Joker, there is no Batman, and without Batman, there is no Joker. The story titled Legends of the Dark Knight explores this wonderfully. Joker thinks he has finally killed Batman as the media begins to report that the Dark Knight is missing. I
nstead of running rampant on Gotham City now that his only obstacle is gone, the Joker instead turns a new leaf. He goes sane and attempts to start a new life, only to regress back to the Clown Prince the moment he discovers Batman has returned. While the Joker doesn't save Batman in this story, he holds onto the hope that the crusader is still out there and seemingly waits for his return, hoping to continue their endless cycle.
1 Fighting Owlman
Owlman ends up traveling to Earth-Prime dressed in the Dark Knight's costume, creating chaos all over the place and hurting his fellow Justice League members. No one but Batman and the Joker know that this other Batman is actually an imposter so Joker agrees to help Batman.
After Joker uses a bazooka to try and stop Owlman (which also happens to be pointed at Batman), the Dark Knight lays unconscious for a moment in the middle of the street as Owlman comes speeding at him in the Batmobile. Owlman plans to run him over but Joker runs in, grabs a grappling hook from the Bat's belt, and swings them to safety.