Fischer-Dieskau is awesome. A modern day rival would be Bryn Terfel. Terfel has a more resonant lower range, but still that silky smooth quality so admired in F-D.
But, IMO, the man who truly had the voice of an angel was Fritz Wunderlich. He is my favorite male singer of the 20th century.
Ficher-Dieskau singing. Note how Schubert uses the piano to imitate the sound of villagers snoring in the night.