What makes a truly great Joker story?
It's a difficult question to answer, as the last few years have provided a veritable glut of Joker stories, many of which feature totally different takes on character. Which isn't necessarily a bad thing, as there's more than one legitimate interpretation, but it makes it pretty clear which writers "get" the Joker and which don't.
Here's my main criteria for a great Joker story:
1) There's a difference, however subtle, between a Joker story and a Batman story featuring the Joker. To be a great Joker story, it has to fall into the former category. Dark Knight Returns, as enjoyable as it is, is a Batman story.
2) Something important has to happen. Not necessarily something earth-shattering along the lines of Robin being killed, but you have to come out of the story feeling as though this was a significant moment in the Joker's career.
3) It has to be grounded in some semblance of reality. The Joker is a villain. He is not a supervillain, except in the loosest sense of the word. And while it's fun to see him occasionally take on Superman or hang out with the Secret Society or whatever, at the end of the day, the Joker is just a man. The craziest, most dangerous man in the world maybe, but still a man. Which is why stuff like Emperor Joker ultimately doesn't work, as far as I'm concerned.
With that in mind, here's what I consider to be the ten best Joker stories ever told.

10. "The Clown At Midnight"by Grant Morrison & John Van FleetBatman #663Plot: Is it possible for the Joker to go even crazier? It turns out, yes, as he begins one of his periodic metamorphoses of personality inside Arkham Asylum, this one making him even deadlier than before.
Why It's Great: I can't recall a comic in recent memory that caused such a schism amongst readers as this did. People seemed to think it was either genius or crap, with very little middle ground. I came down squarely on the genius side. It's probably one of the most ambitious comics in recent memory, a mostly-prose piece that accomplished what Morrison's Arkham Asylum seemed to want to do, which is get inside the Joker's head and show readers exactly what goes on in there. While admittedly, the prose is a bit overdone in places, and the artwork a distraction, this really is an excellent story.
The Quotable Joker: "They all used to ask me, 'What makes the Joker laugh?' and I'd point to YOU. We'd all laugh at you and your stupid bat-toys behind your back."
9. Untitled
by Bill Finger, Bob Kane & Jerry Robinson
Batman #1
Plot: In his first appearance, the Joker goes on a robbery and murder spree across Gotham.
Why It's Great: It's really impressive how fully-formed the Joker is from the very start. All the major character beats are there (personality, costume, Joker venom, etc.), and even though comic books were ostensibly for kids back then, it's pretty clear that the Joker's creators intended for him to be one scary-ass character. And even though the Joker's origin has been retold and modernized numerous times, this story holds up amazingly well almost 70 years later, unlike a lot of other comics from that period.
The Quotable Joker: "If the police expect to play against the Joker, they had best be prepared to be dealt from the bottom of the deck."
8. "The Laughing Fish"by Steve Englehart & Marshall Rodgers
Detective Comics #475-476
Plot: The Joker demands that the Gotham City Copyright Office grant him a trademark on his "Joker-Fish," so he'll get a cut of every fish sale in the world. When he doesn't get his way, he begins murdering government bureaucrats until he does.
Why It's Great: The plot's as corny as anything that ever appeared on the old
Batman TV show, and yet Englehart manages to make it work. In a lot of ways, this was the prototypical modern-day Joker story: Part absurd, part serious, and able to swing back and forth between the two without missing a beat.
The Quotable Joker: "But--what if everybody stops eating fish? I hadn't thought of that! What if they all conspire against me--leave my Joker-Fish in the sea? But no--that would never work! The vegetarians wouldn't go along!"
7. The Devil's Advocate
by Chuck Dixon & Graham Nolan
Plot: After years of skating by on the insanity defense, the Joker finally stands trial for a series of murders, and is given the death penalty. Unfortunately, Batman suspects he might actually be innocent this time.
Why It's Great: The criminal justice system in the DCU isn't something you can really dwell on too much. You read some of those old Batman or Superman comics, and it's bizarre how many times supervillains get released on parole. Really? The guy who's robbed a dozen banks, killed a few people, and joined a bunch of other villains in an attempt to take over the world, now claims he's sorry and is getting
parole? Even in a hippie state like Massachusetts, that wouldn't fly.
Similarly, you can't really put too much thought into why the Joker, whose body count has to be in quadruple digits by now, keeps getting sent back to Arkham Asylum, when everyone knows he'll just escape again. So it's interesting to see at least an attempt at realism here. Plus, there are some great Joker moments, especially his unpredictable reaction to getting the death penalty.
The Quotable Joker: "So you're here to get me off. No double entendres, please."
6. "Soft Targets"by Ed Brubaker, Greg Rucka, & Michael LarkGotham Central #12-15Plot: Gotham comes under siege by a sniper who turns out to be...well, you know. As Batman and the Gotham City cops try and bring him down, the Joker makes their job easier by turning himself in. And that's when his plan really gets started.
Why It's Great: As far as Joker stories go, what makes this even more impressive is that he doesn't really even show up until it's about 3/4 of the way over. But everything leading up to that is gold, as we see how the Gotham City PD and the general public react to a Joker rampage. Even after he's been arrested and is seemingly disarmed, he delivers a chilling reminder of how dangerous he is. In terms of art, Lark really outdoes himself, as his sequence where the Joker steps out of the snowy night right up to police headquarters, is one of the creepiest I've ever seen.
The Quotable Joker: "An iced tea would be fantastic. I don't know about you, but I've always found police brutality really makes you thirsty. Who can say why?"
5. "Dying Is Easy, Comedy Is Hard"by Edward Bryant & Dan Simmons
The Further Adventures of the Joker
Plot: Offended at the idea that anyone other than himself could possibly be considered the funniest person in Gotham, the Joker starts killing off stand-up comedians while they're on stage.
Why It's Great: I'm sort of cheating here, as this isn't a comic, but a story that appeared in a prose collection that came out around the first Batman film. But hey, it's my list. And in a book full of great Joker stories ("Definite Therapy" by F. Paul Wilson honestly gave me nightmares as a kid), this one was my favorite. It has everything you could possibly want from a Joker story, aside from art. But the real selling point? Batman going undercover as a stand-up comic. It just doesn't get much better than that.
The Quotable Joker: "Let us cut right to the heart of the matter, figuratively now, later perhaps literally."
4. "Wildcard!"
by Marv Wolfman & Jim Aparo
Batman #450-451
Plot: Badly shaken after his near-fatal shooting at the end of "A Death in the Family," the Joker is shocked when an impostor turns up, committing crimes in his name, forcing him to come out of hiding before he's ready to.
Why It's Great: One of the more unfortunate trends over the past decade or so is how writers now treat Batman less like a human being, and more like an unstoppable force of nature. Some of this has apparently also spilled over to the Joker. Yes, he's a scary psychopath. But he's still just a human being.
That's why this story is maybe even more refreshing now than it was when it was first published almost 20 years ago. Why wouldn't the Joker, as crazy as he is, be subject to the same confidence problems as everyone else?
The Quotable Joker: "You're more pinstripe than pinhead. Brooks Brothers than Ringling."
3. "A Death in the Family"By Jim Starlin & Jim AparoBatman #426-429
Plot: The Joker (with an assist from the readers) manages to do what no other villain had: kill Robin.
Why It's Great: If any
one of the following things happened in a Joker story, it would probably be enough to make this list. The fact that all
three of them do, is nothing short of awesome. First, the Joker beats Robin nearly to death with a crowbar, and then finishes the job by blowing him to kingdom come. Second, he meets the Ayatollah Khomeini and becomes the Iranian ambassador to the UN. Third, the whole thing is capped by one of the better climaxes of any Joker story ever.
And while the gimmick of allowing readers to decide Robin's fate via telephone vote remains one of the hokiest ideas in the history of the industry, for once, the fans got it right. This story wouldn't be nearly as well-regarded if Robin had actually survived. The fact that twenty years later, real-life supervillain Dan Didio would undo this great story by unwisely returning Robin to life, doesn't negate its impact.
The Quotable Joker: "Prepare yourself for a severe spanking, young man. But let me tell you right from the start...this is going to hurt you a lot more than it does me."
2. "The Joker's Five-Way Revenge "
by Denny O'Neil & Neal Adams
Batman #251
Plot: The Joker suspects one of his old henchmen sold him out to the cops. Rather than find out which one, he figures he'll just play it safe and kill them all.
Why It's Great: Aside from being a perfect story in general, I don't think any artist has ever done a more frightening-looking Joker than Adams. It's also notable for being the first Joker story to dispose of the campy elements of the 60s, and return the character to his roots as a homicidal maniac.
This story also features two of my all-time favorite Joker moments: First, as an exhausted Batman investigates one of the Joker's henchman's apartments, the Joker steps out of the shadows, sucker punches Batman, knocking him out, and could easily finish him off. Then he realizes that would be a hollow victory, and leaves. The other is at the end of the story, when a furious Batman is chasing after him. Admitting that one would have to be stupid (as opposed to crazy) to take on a pissed-off Batman, the Joker flees instead of even trying to put up a fight.
The Quotable Joker: "No! Without the game that the Batman and I have played for so many years, winning is nothing! He shall live...until I can destroy him properly!"
1. The Killing Joke
by Alan Moore & Brian Bolland
Plot: The Joker's origin is told via flashback, while in the present, he goes on a rampage of terror unlike anything seen in a Batman comic at that point.
Why It's Great: Yes, putting this at number one is about as predictable as you can get. But there's really no denying that this is where it belongs.
Undoubtedly the most important Joker story ever told, the "Suggested For Mature Readers" label allowed Moore to get away with murder. Well, attempted murder, anyway, as the Joker cripples Batgirl and mercilessly tortures Commissioner Gordon. More importantly, it's the definitive examination of their relationship, from the beginning of the story, when Batman goes to meet with him in an attempt to get them off the collision course they've found themselves, to the end where, in a moment of lucidity, the Joker refuses Batman's offer of rehabilitation. And if Adams drew the most frightening Joker ever, Bolland drew the perfect, most expressive Joker ever, and is probably considered by many to be the definitive artistic take on the character.
The Quotable Joker: "You had a bad day once, am I right? I know I am. You had a bad day and everything changed. Why else would you dress up like a flying rat?"