I am certain that a significant portion of the material in The Dark Knight Returns (TDKR) will be used in the upcoming DC movie Batman v Superman: Dawn of Justice. So what makes TDKR one of the best Batman comics and in fact one of the most influential comics in modern history? 
 First, TDKR made Bats relevant once again to the public. It made the Batman that we still see today - grim, fearful, badass. The Dark Knight Returns (with Watchmen by Alan Moore) ushered the whole comics medium to the modern era were themes are more mature and serious. Suddenly, comics is not just for kids anymore. The comics medium since then became a respected literary material. 
 Third is the story itself. The Batman versus Superman chapter is my personal second best comics character battle of all time, next to Batman versus Joker at Scott Snyder's Death of the Family. 
 If you ever want to become a serious Batman reader, then you gonna have to start with the big three: The Dark Knight Returns, Year One and The Killing Joke.