Any piece described as "for the profit and use of musical youth desirous of learning, and especially for the pastime of those already skilled in this study" by the composer is going to be interesting at the very least.

And then it turns out that J.S. Bach, the Maestro, the Undisputed First King of the Keyboard himself wrote that, and that's enough said, really.

The Well-Tempered Clavier. Every pianist has played a few. They're standard repertoire basically at any time; I personally get a bit twitchy when I haven't played any Bach in a while. I guess it's a pianist thing.

In any case, the Great Maestro composed two books of "well-tempered preludes and fugues", each containing a work in all 24 major and minor keys, composed to showcase how the instrument can show different colours depending on what key it's played in and because Bach was generally awesome.

I've recorded the first prelude and fugue of the second book - a personal favorite. 

Because that fugue. How can you not love it!