@simonmoon: For a stimulating workout of your brain, give Bach's Concerto For 4 Harpsichords And Orchestra a listen. Each harpsichord plays a separate melody (the form of the Fugue, employing a lot of counterpoint), all following the chord progressions, modulations, etc. played by the orchestra. But it's not just an intellectual challenge, it's also a thrilling musical ride!
For the enlargement of your soul, listen to any of his Passions.
I was introduced to the music of the master by a great songwriter I was recording with in 1974. He had grown up a fan of The Beatles, but when I loaned him my copy of Smiley Smile (The Beach Boys) he became as obsessed with Brian Wilson as was I. That album led both of us to Van Dyke Parks, another near-genius. If you've heard Van Dyke's debut album Song Cycle and like it, there's no reason you can't become enamored with Bach's insanely great music.