One of my very favourite pieces of music, Piano Concerto no. 3 by Béla Bartok (1881-1945), performed here by the City of Birmingham Symphony Orchestra under the direction of Simon Rattle, with Andras Schiff as the soloist:

 

The mini fugue section starting at 0’50, introduced by the piano and then picked up by strings, is sublime.

In fact, I love all three movements of this piece. The second movement is so serene and still (the piece was a gift from Bartok to his wife, written in his final days, and in many ways can be seen as a farewell letter to her, or to life), and makes a marvellous contrast with the explosive start to the final movement, given here.