Recently I went to a birthday party where there was a piano. When asked to play, I had one piece in my fingers, the Op. 118 Intermezzo by Brahms--the Intermezzo isn't party music. That incident reminded me of a piano teacher saying "always have a piece in your fingers." In this case I needed two.
While growing up, I was expected to play when company came to our house, and typically I would improvise "Amazing Grace" or "What a Friend We Have in Jesus". Those were perfect for my father's crowd. Not sure if there's anything in my fingers right now, but if I could sit down at the piano and magically play one work, I'd play Franz Liszt's Transcendental Etudes because of its emotional depth and virtuosic display.
What can you sit down and play or what singular piece of music would you love to be able to play?