I would like to suggest that the big, elaborate picnic scene at Zanadu might be considered the most significant because it is the only place where the film's thesis is actually spelled out in words. The black singer's lyrics, which are interwoven with the action between Kane and his wife in their elegant tent, could go unnoticed, but what he is singing several times is:
It can't be love,
Cause there is no true love.
Kane has no faith in human love because his mother sent him away from Colorado when he was a little boy. This separation and, as it seemed to him, this betrayal shaped his character for the rest of his life.
So many moments in Welles's film can fit be deemed as important. The burning of the sled "Rosebud," the dropping of the snow globe, or even the loud whisper of "Rosebud" as Kane's final words could all constitute as significant scenes from the film. In seeking to enhance the idea that any scene from Citizen Kane is groundbreaking, I would suggest the scene that interrupts the opening of the film is significant.
When the audience is left with Kane died and his last words, the viewer is abruptly taken to a film room of Kane's life. The media has seized on the story. Thompson is seen wondering openly about "Rosebud." In the deep focus shot, the viewer sees footage of Kane's life, as well as the media seeking to make a story about it. Adding to this is how the viewer never really sees Thompson. We simply hear his voice and the demands of the media wanting to know more is almost akin to a faceless voice that simply craves more without anything else.
Welles's genius is seen in this scene in a couple of ways. The first is that it shows how there is a celebrity consumption process in modern society that refuses to restrain itself. Death does not stop this machine, but actually accelerates the process. In never being able to see Thompson, Welles develops this mechanized aspect of the media. The media simply wants to consume something else. The deep focus shot helps bring everything into this moment, a reflection of how the mass consumerism of society reduces and dehumanizes everything in its path. As we understand more about Kane himself, this scene gains even more significance as Kane, himself, helped to create this machine that devours even him in death. Another reason why the scene is so important is that it shows how the modern setting is one where information is present, but truth is absent. Thompson is shown to simply want to assemble information. He might claim to want to know more about "the truth" about "Rosebud," yet there is nothing transcendent being sought. It is a scene that helps to illuminate how the modern setting has brought about greater access to information, but little in way of paradigm to understand it in a meaningful manner.
This opening scene regarding the media is important because it establishes both the technical structure and thematic relevance of the film. Thompson's assembling of insight and facts about Kane's life starts from this scene. We see the desire for information and atomized bits of knowledge, with nothing meaningful grasped. Thompson embarks on his quest in this scene, only to concede that he is nowhere farther in its understanding at the end of the film. At the same time, Welles is able to use this opening to help assemble the plot narrative of Kane's life. As Thompson assembles information and he comes to know Kane, so does the audience. Through this, Welles is able to bring about reflection from the audience about the media and its role in understanding celebrity. This scene acquires importance on both thematic and storytelling levels.