Schubert's Impromptu Op.90 No.3 in Gb Major.
I've never really found this piece all that interesting when I was younger but with time, the apparent simplicity of this piece revealed itself as something much more profound than I had originally thought.
The opening bars could hardly be simpler but it's exactly that which makes it almost surprising to the listener, the Eb minor chord colouring the same note differently after three iterations of the same Gb major chord.
Of course, Schubert is a master of melodies and his supporting harmonies take surprising turns in different tonalities while always maintaining a careful and perfectly balanced discourse.

Recording (Horowitz):    • Vladimir Horowitz    Schubert Improm...  
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