I've had Fischer-Dieskau's recording of Schubert's Winterreise cycle for a long time. Listen to it about once a year. His voice and Gerald Moore's piano. Beautiful stuff.

Last year I picked up an 11-disc set of Fischer-Dieskau, a retrospective on EMI. All the major song cycles and assorted masterpieces. I've heard voices that I might like better, but I can see why some people say he was the greatest singer of the twentieth century. An amazing voice.

As far as his folk ties, a lot of the German art songs were derived from folk sources.