Tony Stark was the man who started it all on the big screen for the Marvel Cinematic Universe. In 2008, Marvel wanted to create a shared universe, and the character they gambled on was an instant success, as Iron Man became the backbone of the first 11 years of MCU movies. By 2019, Tony helped change everything about the Marvel Universe, and then he died a hero. With his death, a giant hole opened in the MCU that will be almost impossible to fill, however, his legacy will continue.
A lot of people believe Tony's legacy is his work as Iron Man, and that whoever steps into that role will be the one to carry the torch. Rhodey is already behind one of the suits as War Machine, and if Riri Williams enters the MCU, she could also take on that challenge as Ironheart. However, it isn't essential to find someone to replace Iron Man since his legacy goes beyond what he did in the armor. Tony's legacy lies with the kids he helped throughout the films: Peter Parker, Harley Keener and Morgan Stark.
Harley Keener
Harley is the boy that Tony met in Iron Man 3. He was a fatherless child in Tennessee who lived with his single, hardworking mom and had a sharp mind. When Jarvis took over and flew Tony to safety after AIM's attack, he landed in Tennessee. Tony needed a place to lay low, and Harley was there to help him, so Tony was there for Harley as well.
Harley was in a rough spot, with bullies pushing him around. It was his relationship with Tony, and vicariously Iron Man, that helped him overcome his insecurities and discover himself. By the end, Harley helped Tony get his head back in the right spot thanks to basic childhood logic. Tony later repaid Harley's kindness with a shed full of technology to help the young man accomplish anything his mind could come up with. While their time was short together, Harley attended Tony's funeral in Avengers: Endgame to honor the man that made a significant mark on his life.
Peter Parker
Peter was already a superhero before he met Tony as the Friendly Neighborhood Spider-Man, who stopped minor crimes and often found himself at odds with authority. Despite his superpowers, Peter was never self-assured, regardless of the quips and insults he threw at villains. Part of this comes from not having a father figure and blaming himself for the loss of his Uncle Ben, but then Tony stepped in.
The one thing the MCU gave Peter that no other theatrical movie supplied him with was a father figure in the form of Tony. Much like Harley, Peter needed someone to push him to reach his full potential. In Spider-Man: Far From Home, Peter said he didn't know how to go on without Tony, but proved that what Tony gave him in their short time together was enough, making Peter an even better hero.
Morgan Stark
Tony never had a dad in his life. Even when Howard Stark was alive, he had little time for Tony and was always working. As an adult, without ever meaning to, Tony himself became the father figure he needed for Peter and Harley, seeing a lot of himself in both boys. The man who never had a dad became one for two kids who needed one, and then he became one for his own child, Morgan.
After Thanos wiped out half of humanity, Tony chose to be a father to his daughter rather than immersing himself in trying to save the world again. Being the best father to his child is its own form of heroism. When the time came, Tony returned to his superhero identity as Iron Man, and this time gave his life to save the world and his daughter, but before his death, he did something his dad never did. He provided his little girl with a father figure, teaching her what it means to be a hero and a loving parent.
Tony was brazen, brash, self-assured, arrogant and selfish at times, but he was a hero in the MCU who left a huge mark. However, Tony's real legacy is what he meant to the people he helped, like these three kids. They all needed someone to look up to, and he gave them the confidence they needed. What Tony did for Harley, Peter and Morgan is just as important, if not more so, than what he did as an armored superhero.