My all time favorite Sergio Leone movie is 'Once upon a time in America'. (1984)
In my opinion it is the best gangster movie made.
With Robert De Niro and James Woods in the leading roles. Playing childhood friends, who grows up to become gangsters.
'As boys they said it, as men they did it'.
The movie is based on the 'The Hoods' by Harry Grey.
Sergio Leone said himself he devoted most of his adult life creating it.

Leone turned down the opportunity to direct 'The Godfather', in favor of working on another gangster story he had conceived before the offer of The Godfather. Leone devoted ten years on this project, based on the novel 'The Hoods' by Harry Grey, which focused on a quartet of New York City Jewish gangsters of the 1920s and 1930s who had been friends since childhood. The finished film, 'Once Upon a Time in America' (1984), starred Robert De Niro and James Woods. It was a meditation on another aspect of popular American mythology, the role of greed and violence and their uneasy coexistence with the meaning of ethnicity and friendship. The studio cut (only for the American market) its four-hour running time drastically, losing much of the sense of the complex narrative. The recut version flopped and received much criticism.

The original version, projected in the rest of the world, received great appreciation by the public and by critics.

When the integral version of the film was released on DVD in the USA, it gained major critical acclaim, with many critics hailing the film as a masterpiece.

The score for the movie was made ten years before Leone even started making the it. Leone and Ennio Morricone agreed on cues and what music they wanted to quote in the movie, and amazingly enough most of that music stayed in the film, as it was initially imagined back in the 60's.
Leone and Morricone were old classmates and has worked together on most of Leone's movies.
Besides the music he did for Leone's movies, he has, among others, made the music for Brian De Palma's 'The Untouchables'.
In 2007 Morricone won the Academy Awards' Honorary Award.

In 'Once Upon a Time in America' the young version of Deborah is played by Jennifer Connolly - Who is still a known actor.
This was her first actual movie appearance.

Leone would often show the actors how he wanted particular things done.
Like showing Clint Eastwood how to draw his gun or tip his hat.
This was a part of Leone's manner of directing - he would leave nothing to chance.
On the photo, Leone is showing the cop how to handle Noodles.

The trilogy consists of 'A Fistful of Dollars', from 1964, 'For a Few Dollars More', from 1965 and 'The Good, The Bad and The Ugly', from 1966. All three starring Clint Eastwood as The Man with No Name.