Batman Beyond came out in 1999, the fourth show in the DC Animated Universe. What was intended to be a "kid friendly" version of Batman instead became a shockingly mature, developed, and admirable entry into the Batman mythos. The idea of "Let's make some teenager Batman and keep Bruce in the cave" could have been a disaster, but Terry McGinnis serves as a worthy successor to Wayne.
It feels like a natural extension of the DC world. Bruce keeps up the fight until he's forced to break his one rule, and he never forgives himself. Neo-Gotham is such a fleshed-out world that feels like what the DCAU would be like decades after the fact. There are roving criminal gangs who style themselves after the Joker. Slappers are a popular recreational steroids derived from Bane's venom compound. Barbara Gordon follows in her father's footsteps as Commissioner. The Justice League is still around with a new roster, and Wayne still keeps them at a healthy arm's length.
The cyberpunk setting of Neo-Gotham is phenomenal. Slang like "schway", the design of the city, the advanced "hockey" played at the high school, it's very well done. Terry's Batsuit is iconic, and a flying Batmobile with all the tech that comes in the distant future of 2039 does so much to make it a unique take on the DC world. Not only are there returning villains like Mr. Freeze and Ra's al Ghul, but Terry gets his own rogue's gallery with characters like Blight and Inque, who become iconic in their own right throughout the series.
Terry himself is such a fantastic character. A teenage Batman could have easily become a Peter Parker knockoff, but the way his character develops and he manages being Batman by night, as well as being a high school student with friends, romantic interests, and a family do a lot to make him his own character, and in time you really do come to view him as Bruce's legitimate successor.
The Return of the Joker movie is one of DC's best, and provides such a dark final chapter to that era of Batman. Dick went off to another city, Tim was tortured and disfigured, Barbara was basically forced by Bruce to stop being Batgirl, and Bruce kept it going on his own. For years. Only for Joker to pop up again before finally being put down for good. With Terry handing Batman's greatest villain his final defeat, he truly becomes Batman.
With the glut of DC movies we've gotten in the 25 years since Batman Beyond ended, I'm genuinely amazed that we haven't gotten a movie or something, or at least some sort of multiverse appearance by Terry. It's such a rich world that could easily be expanded upon. For a cartoon airing on Kids' WB, it really did punch above its weight in a lot of ways, and is to many people the definitive future of the Dark Knight.