Iron Man faced his fair share of worthy villains in the Marvel Cinematic Universe, but the most important was Ultron rather than the Mad Titan Thanos. Tony Stark is undoubtedly one of Marvel's greatest heroes, and he dealt with plenty of adversity, going up against antagonists like Iron Monger, Whiplash, Justin Hammer, Aldrich Killian, and Loki. Iron Man also battled inner demons throughout his journey and certainly came a long way before making the ultimate sacrifice to snap Thanos and his army out of existence using the Infinity Stones.

Tony's journey towards this selfless decision featured many classic comic book foes who have made their appearance on screen, but there is one villain Iron Man faced who stands above the rest in terms of their impact on both the hero and Tony Stark himself. The mechanical menace known as Ultron became Stark's greatest personal failure in the MCU, given his central role in creating the villain, whilst Thanos served as a reminder of the potential consequences should Tony ever choose to go down that road again.

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Without the failure brought about by the creation of Ultron, it is likely that audiences would never see Iron Man become the hero he does by the finale of Avengers: EndgameIn fact, the creation of Ultron represents all the worst aspects of Tony Stark's character, and his overcoming of Ultron represents the beginnings of his greatest victory and sacrifice. This makes the A.I. Iron Man's most important villain.

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Avengers: Age of Ultron is a film about the manifestation of Tony Stark as a villain. His refusal to accept the blame for Ultron's creation, his blatant disregard for the role of his superhero allies, and his desire to push past safety measures in order to get a closer look at alien technology all culminate in a devastating villain and countless deaths. Iron Man is very much presented with himself as a villain, and it comes about because he tries to do what he believes is right for the world based on his own flawed concept of perfection. The vision that Scarlet Witch bestows upon Iron Man at the start of Avengers: Age of Ultron reveals to him his worst fear and sets him on a path to prevent a world that has not happened yet.

Throughout the entirety of the movie, and right up until the conclusion of Avengers: Endgame, Iron Man is still learning the lesson that begins in Avengers: Age of Ultron: being a hero is as much about saving the day as it is about accepting the tragedies that happen along the way and handling the responsibilities that the role entails. His journey throughout the MCU after Avengers: Age of Ultron is heavily focused on fixing past mistakes, but Iron Man's lasting legacy in the MCU is so much more than that.

Almost the entire Infinity Saga allows Iron Man to reflect on his own mistakes, and many of his subsequent MCU appearances feature character arcs where he tries to undo previous failings. His appearance in Captain America: Civil War very much shows his desire to make amends for Sokovia, and his Spider-Man: Homecoming appearance features him telling Peter Parker that he wanted the Webhead to be better than he was. Iron Man's experience and failure with Ultron paves the way for him to continue doing what he believes is right, but without the arrogance and brashness that dominated his journey up until that point.

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By the time Avengers: Endgame comes around, Tony has accepted his failure and has built himself a family life with his wife Pepper and his daughter Morgan. Yet, he still understands that his one true chance at redemption lies in giving up everything to undo a mistake and make the world a better place, even at the cost of his own life. A lesson that began with Ultron was reinforced by his encounter with Thanos and ultimately ended up with the MCU's greatest sacrifice. The importance of Ultron cannot be overstated in Iron Man's journey, and the result of Avengers: Endgame may very well have hinged upon Iron Man recalling that lesson in that most important of moments. Whilst it may Iron Man's encounter with Thanos that most audiences remember, it is his journey with Ultron that forges him into a hero for the ages.

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