Night is full of moments that stick with readers, and the scenes that seem "important." For me, I think about themes in the book and moments that illustrate those themes. One theme is the surreal nature of what has happened to Eliezer; his experience traveling past his home inside a boxcar bound for a concentration camp, and his desperation to catch a glimpse of a familiar place, is a detail that captures something of his psychological state. His arrival at the camp and the sight of the chimneys of the crematorium is another hellish moment in which the reality of his experience seems almost beyond comprehension.
A second theme would be the humanity of the prisoners, even in the face of extermination. Take, for example, the story of how Eliezer is secretly befriended by a French woman after he is beaten at his job in the factory: the woman offers words of encouragement to him, even though she knows that if the SS knew she spoke German, she would likely be sent to the camps.
A third theme would be the will to survive no matter the cost. One example that stands out is the time during an air raid when a prisoner defies orders and crawls from his barracks to an unguarded cauldron of soup. It is an act of desperation that every prisoner vicariously participates in; Eliezer watches his progress closely, only to see him shot and killed the moment he reached the cauldron.
There are many such moments. The strength of Wiesel's book is in the clarity of its detail.
In a question like this, it seems to work out best if the reader is able to determine what constitutes importance and the selection of scenes is based off of this. In Wiesel's work, there seems to be so many important moments where so many truths are revealed about Eliezer's belief systems and his experiences in the Holocaust. One could argue that the moment back in Sighet, where the villagers reject Moshe the Beadle's claims about what he experienced would be a critical moment because it reflects the denial and silencing of voice on the part of the victims which helps to embolden the aggressors. At the same time, Eliezer's experiences at Birkenau could be deemed as "important" because Eliezer rejects the presence and benevolent force of God when he sees babies and children incinerated at the hands of the Nazis. It is also an important moment because Eliezer is separated from his mother and sister. While there are so many important moments, I would think that the child's execution at Buna could be another important moment because it is at this moment where Eliezer's complex approach to the divine is revealed in the quotation, "God is in the gallows." Again, to find three important scenes is both easy and difficult because there are so many moments in the essential realm and difficult to make a choice from such powerful ones.