Johnny Fontane

This is the scene credited for the " I am going to make him an offer he can't refuse" spoken by the Don which has become the quote of the film. Although spoken differently in the novel, the line to me has more to do with Vito Corleone displaying his conviction of resolving all matters in a favour first, escalate to violence later manner. But with the understanding that violence is inevitable.
The scene itself was not so much aobut Johnny Fontane but more about Vito's Corleone's feeling of some contempt for his working sons in his Business. Asking Johnny if he spends time with his family to display his distaste for Sonny's choices. It is apparent that Tom Hagan is no fan of Johnny from the opening of the scene "Its been two years..he is probably in Trouble" and he also openly smiles as Johnny gets told "You can act like a man". Nothing goes unnoticed with the Don, he sends Tom to go talk to Waltz almost as a punishment for mocking one of his favourites and showing up Johnny by smiling. The way the scene is shot it establishes Tom as the "son" on the outside and Sonny as the son not focused on what needs to be done. This scene to me is the first scene of the film, while the "I believe in America" and "Wedding" were chapters meant for the reader or the audience of the film.