I echo Beethoven Opus 28 (a tremendous composition!), the WTC Book 1, Mozart K.576 and Rachmaninoff Etudes-Tableau.
Also
Rachmaninoff Prelude Op. 23 No. 4, breathtakingly beautiful
Scarlatti Sonata K.491, WOW, this is seriously one of the most fascinating compositions I have ever heard, especially when played by Trevor Pinnock!
Haydn Sonata No. 37 (I believe, it's one of the most famous ones, and one of the best)