5. BATMAN: THE KILLING JOKE by Alan Moore and Brian Bolland (1988)

I’m sure many who read this list will think to themselves that this graphic novel is better than some of the later books on this list, and you’re probably right. But Joker’s origin is only one piece of The Killing Joke. The rest is about cruelty and dissocation, and it’s a no-holds-barred picture of Joker as an evil, despicable human being—and one who nevertheless Batman allows to live.
Still, as origins go, it’s the best (and most definitive) Joker origin since Detective Comics #168 introduced the “Red Hood” backstory. DC swears they won’t do to Joker what Marvel did to Wolverine by giving a linear, clear, canon-precise Joker origin—so this is the closest we can come to.

And in case you don’t recall what the killing joke was….

killing joke