One of the most popular is 'The Dark Knight Returns'. It's popular because it presents a different take on Batman, an older and more jaded version. It also influenced a lot of the Batman movies we see today. 'Batman: Year One' is also very popular as it gives a great foundation to Batman's story. It's often recommended for new fans to understand where Batman came from.
The Killing Joke is extremely popular. Its exploration of the Joker's origin and his relationship with Batman has fascinated readers for years. It has some really iconic moments. Also, 'Batman: Long Halloween' is well - loved. It has a great noir - like atmosphere and a complex mystery that keeps readers engaged. It shows different villains and how they interact with Batman during different holidays.