1. Casting Robert Downey Jr. as Tony Stark
Downey Jr. was struggling to return from a long spiral of drugs, arrests and shaky career choices when he landed the role of the billionaire weapons manufacturer who turns into an armored defender of the world. Since then, he and Tony Stark have been almost interchangeable: It's hard to think of another role in recent movie history that's become so closely associated with one actor (good luck trying to recast the character after The Avengers 3). Downey's charisma, cockiness and brilliant timing helped turn Iron Man an instant classic, kickstarting the Marvel Cinematic Universe and making him the superstar he was always supposed to be. The Age of Marvel really, truly starts here —and with him.