What is the most influential quote in Night about losing identity?
Clearly the loss of identity is a massive theme of this excellent novella, and one part that you could definitely use is when Elie is tattooed and given a number and has his name taken away from him. This is a significant moment for all Jewish concentration camp survivors as it is another step of their original identity being taken from them, and allows them to be referred to as just a number rather than a name, and thus dehumanised. Note what Elie says about the experience himself:
In the afternoon, they made us line up. Three prisoners brought a table and some medical instruments. We were told to roll up our left sleeves and file past the table. The three "veteran" prisoners, needled in hand, tattoeed numbers on our left arms. I became A-7713. From then on, I had no other name.
Notice how Elie says he "became" the number, and from this stage on in the narrative, he "had no other name." Obviously this is a significant moment in terms of the theme of loss of identity in the novel and such a stage is part of the process created by the Nazis in stripping their prisoners of their identity.