Source: lego.com
6. There Are No Joker Stories, Just Batman Stories
Again, this is a generalization not a direct statement of fact but “there are almost no Joker stories and the ones that technically qualify are pretty useless†makes for a worse entry title.
But yeah, most of the big Joker stories are actually Batman stories. The Killing Joke? A Batman story (and Alan Moore’s worst DC story, as well, according to both me and Alan Moore) Joker’s Five-Way Revenge? Total Batman story. The Man Who Laughs? 100% a Batman story.
Even stories that are more about The Joker then they are about Batman use another character as the actual main character. Mad Love is focused around Harley Quinn. Joker’s Favor, which is the closest thing to a Joker story that I can think of being good is focused around the normal person drawn into Joker’s madness.
Everyone loves the Joker but Joker stories? They need a different focal point.

