One great 'Joker x Reader Fanfiction' is 'The Darkest Temptation'. It delves deep into the complex relationship between the Joker and the reader. The author does an amazing job of portraying the Joker's unpredictable nature while also developing a connection with the reader character. Another good one is 'Joker's Obsession'. It shows how the Joker can be both terrifying and oddly alluring in his pursuit of the reader.
Well, 'Shadows of the Joker' is a must - read. It has a very atmospheric setting that draws you into the world of the Joker and the reader. The character development is excellent, especially for the reader character who has to navigate the dangerous waters of being involved with the Joker. 'The Joker's Whims' is also quite interesting as it shows the Joker's capricious nature and how the reader tries to deal with it.
I really like 'A Dance with the Joker'. In this fanfiction, the story is set in a dark and gritty Gotham. The way the writer describes the interactions between the Joker and the reader is really engaging. It's full of suspense and keeps you on the edge of your seat. There's also 'Joker's Prey' which has a unique take on their relationship, with the reader being a bit more of a challenge for the Joker.