What is the one thing you don't really need?
You don't need most things. What you need is a shorter list:
Biologically you need food, water, oxygen, and a temperate environment.
Next in regards to mental health, you need shelter/home, any social interaction, purpose, sex, your health, and daily routine.
Then more specific, a nice home, a good paying job, a happy family, and close friends.
After that is basically a good wifi connection and porn.
Okay, having read a bunch of these, they go many different directions, I'll just go light and state two belongings that I do care about, but don't really need, but would miss. My Funko Pop collection. 125 and counting. My physical books. I never read them anymore, and though I lost many tens of thousands of them a few years back, I have about 200 probably. I listen to audible books and read on the Kindle. So I don't need them. Also, library.
Of course, at this point, I have no intention to get rid of these things. But I don't need them.