Jack Nicholson — Almost Michael Corleone

Jack Nicholson nearly played Michael Corleone in The Godfather, but insisted the role go to an Italian-American actor, a selfless stance in 1970s Hollywood. The film grossed $245 million (over $1.5 billion adjusted for inflation) and made Al Pacino a legend. Nicholson went on to win three Oscars, so this isn't a career regret, but it's a profound "what if." He is 30% more decorated than the runner-up on this list in terms of Oscar wins. Unlike vague refusals, Nicholson's pass was principled and pre-emptive, outperforming #9 Tom Hanks' Jerry Maguire choice because Hanks had already secured his legacy with two Oscars, while Nicholson's would have altered cinema history.