Am Tage aller Seelen (Litanei) D.343 Franz Schubert
It's the keyboard work that has lasted through the centuries - Bach's Well-Tempered Clavier is the aspiring pianist's go-to text.
These works are also collectively known as The Well-Tempered Clavier, which as the name suggests is a collection of solo keyboard music composed by Johann Sebastian Bach. The 48 Preludes and Fugues are regarded as the most influential works which ultimately revolutionised western classical music. The Well-Tempered Clavier comprehensively reassessed the approach to not only the playing the keyboard, but to composition in general.
Constantly in rotation on the music stands of all aspiring pianists at one point or another, this legendary collection of keyboard nuggets is perfectly formed not only within each movement, but overall. It explores every aspect of the ebony and ivory (or whatever harpsichords were made out of in the 17th Century), and continues to provide challenges to each new generation.