Chopin's Preludes, Op. 28, easiest first
Frédéric Chopin's 24 Preludes, Op. 28, published in 1839, go through every major and minor key. Each is short, some under a minute, and each has one mood: a quiet dance, a slow procession of chords, a storm. They are among the most played pieces in the piano repertoire, and several are well within reach of an intermediate player.
The preludes in Octotune are graded 5 and 6 of 8. The usual first ones are the A major, No. 7, and the E minor, No. 4. They are listed easiest first; each page gives its grade, key and time, and how to practice it.