Different interpretations of the word "important" play a part in answering your question. That said, consider these two scenes from the play. The scene in Willy's Boston hotel room is important because it explains a great deal about Biff's unfocused life, his continuing hostility toward his father, and Willy's repressed guilt. When Biff finds his father with the woman in the hotel room, Biff's respect for his father is shattered and their relationship is forever altered. Biff stops respecting Willy, but he can't stop loving him; as a result, Biff is thrown into a personal conflict that largely shapes his life from that point forward. This scene is important because it adds so much to the development of the characters and conflicts in the play.
Another scene is important for a different reason. Willy's imaginary conversation with Ben in the play's conclusion is important because it reveals the final disintegration of Willy's mind. Willy tries to "sell" his brother on the idea Willy should kill himself so that his family can collect the proceeds from his insurance. By the end of this dialog, Ben seems to be in agreement with Willy. From this we infer that Willy will carry through with his planned suicide, which he does. The scene with Ben is important because is shows Willy's desperation to "succeed" financially and because it foreshadows the play's conclusion.
I'm surprised that no one has yet mentioned the scene in which the title of the play is mentioned. Willy goes to Howard's office to convince Howard to give him a raise and to take him off the road. Howard thinks that Willy, like all others, should pull their own weight in the business. He isn’t concerned with Willy’s plight—all that matters to him is money and business. Willy tells Howard about Mr. Singleman, the ultimate salesman, who died a respectable death and was remembered and well known throughout his district. Willy is in a dreamy state here as he recalls this story, suggesting to the reader that he in fact wants to live a life as a well-known and respected person.
Usually the important scene is the one in which the truth is revealed. When Biff gets to know that his way in life by following his failure father is false, so he tries to change and tell Happy the son who is greatly admire his father that Willy 'don't know who he was' Biff in confident said: 'I know who I am, kid'
Arthur Miller's "Death of a Salesman is an iconic staple of English high school classes.
The most important scene is near the end when Happy is extremely upset when Biff says that Willy "didn't know who he was." Biff has achieved an anagnorisis, a recognition, for the truth about himself when he says," I know who I am, kid."
Biff has won the right to express the truth about Willy. Happy, on the other hand, is very much like his father and vows to follow in his footsteps. Happy can not admit that Willy's dreams were "all, all, wrong."
The most importnat scenes are when the truth is revealed. That is why I think this was the most important scene in the play.