Let me ask you a simple question. Did you hear the name, Iron Man before 2008? I didn't, and many of you will agree with me. So to turn a seemingly unknown comic book superhero even in the US to one of the most popular characters in cinematic history must have been quite a challenge. Marvel achieved the unthinkable by giving it a strong character arc.

When we first meet Tony Stark in Iron Man (2008), he is arrogant and an egoistic billionaire. He already had a significant number of inventions to his name. However, when he is kidnapped using the same weapons for which he takes credit, he is hit hard. So after he escapes using Mark-1, he goes into redemption. He closes the weapon manufacturing division of Stark Industries, improves the Iron Man suit and goes on righting some of his wrongs. He starts taking responsibility and accountability of his decisions. That's a striking change from the happy go lucky attitude at the beginning of the film.
Next in Iron Man 2 (2010), we see him struggling with his powers. This constant struggle within himself made the character more relatable to people, who were all tired of watching a perfect superhero with no issues.This movie also started a vital arc of Tony's relationship with his father, which told us why he was such a brat earlier. Further movies explored this relationship more.

Avengers (2012) was a landmark in his journey. In the beginning, Captain America labelled him as selfish, with him being unable to take one for the team. The picture ended with him almost sacrificing himself for the planet. This movie also made Tony realise that Avengers might not be able to save Earth every time which led to his PTSD in Iron Man 3 (2013) and the making of Ultron in Avengers: Age of Ultron (2015).
Captain America: Civil War (2016) made me realise the brilliance of Tony's arc. He rejected the idea of government intervention in Iron Man 2. In Civil War burdened by his actions of making Ultron and almost leading life on Earth to extinction, he realised that working with the government would keep him and the rest in check. It also had a brilliant twist of Bucky killing Tony's parents. Tony's reaction at the end made the audience realised that despite his troubled past, he loved and cared about his parents. Spider-Man Homecoming (2017) didn't have many scenes of him but had a significant line. When Tom Holland's Spider-Man told him that he wanted to be like him, Tony replied " and I want you to better. " This dialogue further reassured the fact that he is not perfect and has his shortcomings.

Avengers: Infinity War (2018) focussed hugely on Iron Man. He was the only avenger which Thanos knew and for which he had respect. Thanos recognised that like him Stark was a futurist and wanted to save the world, but their ways were different. Tony was in an emotional turmoil at the end of the movie, with not only losing to Thanos, who had been in his head for years but also losing Spider-Man, who was his closest apart from Pepper.He was so broken that when in Avengers: Endgame (2019), the rest told him that they had a chance to change whatever happened, he declined. But he couldn't rest until he undid the whole thing. That's why I think that Tony Stark received the perfect ending. He wouldn't be able to rest because one thing would come up after another and he would keep donning that armour.
I have always believed that MCU is just a TV series on the big screen. The way that Iron Man's character was handled and developed is just proof of that. He is now in pop-culture and is more popular than Han Solo. Not a bad feat for a second-tier superhero.