Here is a pretty darn good singer I didn't know until I viewed the Stefan Zucker's documentary Opera Fanatic: soprano Marcella Pobbe. A contemporary of Callas and Tebaldi, she sang from 1949 into the early 1970s, in some of the world's great opera houses, with great conductors and singers. You can see why from these video clips.
Adriana Lecouvreur, Io son l'umile ancella, RAI movie from the 1950s (unnamed cond.):
Tosca, Vissi d'arte (Orchestra Sinfonica di Torino della RAI, cond. unspecified)
A live performance of Tosca, Genova, 1965. Toward the end of a long, long career.