Der Ring des Nibelungen is a combination of four operas created from 1848 to 1874 by German composer Richard Wagner (1813 - 1883). The four operas are Das Rheingold (The Rhinegold), Die Walküre (The Valkyrie), Siegfried, and Götterdämmerung (Twilight of the Gods), and they are meant to be staged on four consecutive evenings. The story is a continuous one and I tend to listen to all four in sequence from CD within a few days. A clear example of "the sum is greater than the parts", so I decided to include the complete Ring as one entry. For more information on the four operas I refer to the Wikipedia entry. In general, I've found opera a tougher nut to crack than most classical music genres, but this is the one that did it for me. The complete cycle typically lasts 14-15 hours. I have four versions, of which I have a preference for the legendary box by the Vienna Philharmonic under Georg Solti with numerous famous soloists on Decca/London.
