The Avengers star revealed that the first instalment in the comic book franchise helped him get his career back on track.
"The first one changed everything for me," Downey Jr told the Los Angeles Times. "And with the second Iron Man there were certain aspects that were dissatisfying and disappointing to me but at least they lit me right."
He continued: "[The first one] was a meditation on responsibility and an exploration of how a small group of people can take a two-dimensional idea and, if the winds are right, create something that makes people say, 'That was my favourite movie of the year'."
Downey Jr went on to speak about his decision to sign on to Tropic Thunder, for which he received an Oscar nomination in 2009.
"I signed up to do it and then I did Iron Man and I thought, 'You know, I think I just buried myself. I did a movie that is going to re-establish me here in a little way and now I'm going to squander any goodwill I have'," Downey Jr admitted.
He concluded: "This movie ended up being the most cathartic thing I had done in ages and one of the happiest times in my life."
The actor is reportedly looking to take on the lead role in a big-screen version of Perry Mason.
The Avengers' full trailer is expected to debut next week.
Downey Jr will next be seen in Sherlock Holmes: A Game of Shadows, which is released on December 16.