Batman Vol. 1 #251 "The Joker's Five Way Revenge!"
Batman Vol. 1 #251 is one of the most important issues, in my opinion, in comic book history. The story follows The Joker, having broken out of prison, as he makes good on a murderous vow of revenge. The Joker is at the top of his game in this issue and, when he renders Batman unconscious, is given the choice whether or not to kill his arch enemy...
Superman: Emperor Joker
In Superman: Emperor Joker, The Joker steals 99% of Mr. Mxyzptlk's power and uses it to recreate the universe in whatever way he likes. Emperor Joker spends some time playing around with power dynamics (transforming the real world Justice League into criminal versions of themselves, etc.) but the real story here has to do with Batman. The Joker uses his cosmic powers to literally torture Batman to death every day... and Superman is forced to watch as his friend is devoured by Vultures and killed/rekilled by Zombie clowns. It's a fun book.
(New 52) Batman #38 "Endgame pt. 4"
Batman #38 is an installment of the "Endgame" story arc, written by Scott Snyder with art by Greg Capullo and Danny Miki. "Endgame" follows The Joker's return to Gotham, the first since the "Death of the Family" story arc in 2012, and his attempt to finally end his "relationship" with Batman. This arc offers a lot to Joker fans by way of new origin details and even immortality.
Arkham Asylum: A Serious House on Serious Earth
Arkham Asylum: A Serious House on Serious Earth, written by Grant Morrison and illustrated by Dave McKean, is an important offering because it is rumored to be the book that inspired Heath Ledger's depiction of The Joker.
Batman: Shadow of the Bat #37
Batman: Shadow of the Bat #37 follows the story of Wilde Norton as he hunts down The Joker in an attempt to get revenge for the murder of his family. Norton is given files by Jeremiah Norton regarding The Joker's time at Arkham Asylum. Norton begins studying these files religiously and goes kind of crazy with his obsession. As Norton flips through The Joker's file we, as readers, are given a lot of information into The Joker's mental state and origin:
"In my dream the world had suffered a terrible disaster. A black haze shut out the sun and the darkness was alive with the moans and screams of wounded people. Suddenly a small light glowed. A candle flickered into life, symbol of hope for millions. A single tiny candle, shining in the ugly dark. I laughed, and blew it out."
The story of #37 is pretty slow, serving mostly to give insight into mind of The Joker, as his goons kidnap citizens from all over Gotham. InShadow of the Bat #38 we learn that the kidnapped citizens were chosen for a reason- a special comedic performance with a deadly twist.
Batman: The Killing Joke
Batman: The Killing Joke, written by Alan Moore and illustrated by Brian Bolland, is one of the most popular Joker stories ever written. The story follows The Joker as he kidnaped James Gordon in an attempt to prove that, with a little push, even a good man can be driven to madness. The Killing Joke is also widely accepted as the official origin story for The Joker, as he remembers failed comedy shows, early criminal acts, and the mutation of his body into his iconic clown-like countenance.
Batman #1 is the first appearance of The Joker. The book depicts the clown prince of crime as a homicidal murderer that takes pleasure in finding new ways to cause suffering. This book is especially important because it showed The Joker's character as dark as it was originally intended- before the Comic Code Authority regulated the content of comics and watered down any truly villainous content.
The Clown Prince of Crime
In the 1970's The Joker had his own comic book series which lasted for only 9 issues. Those issues are collected in a book called The Clown Prince of Crime and show The Joker taking on heroes and villains alike. While the book doesn't show the dark version of The Joker that I usually like to see- he does murder a number of people, come up with some sinister plots, and laugh wildly about throwing a man into an incinerator.
Batman: A Death in the Family
Batman: A Death in the Family shows just how ruthless The Joker can be. I won't say more because you should really read this one, if you haven't already, and know that the public (real people, fans of Batman) let the story unfold the way that it did.