The most wonderful things I have ever found on YouTube are just audio: two versions of Kirsten Flagstad singing the Liebestod from Tristan und Isolde.
One is a recording of her Covent Garden debut in the role of Isolde in 1936, Reiner conducting.
The other is a studio recording also at Covent Garden in 1952, Furtwaengler conducting.
Both are obviously Flagstad. No one sounds like Flagstad. No one does that wonderful Flagstad slide.
I have always loved the story, retrieved from the Met archives if I recall correctly, that when the Met had contracted her to sing Wagner, she had never sung him. They sent her off to Hungary (?) to coach with the young Solti. He said she would do fine. This is my all time favorite understatement.
The extreme delicacy of the early recording is fascinating. She is fully halfway into it before she begins to bring any power into it.
The Flagstad I have known and adored is the woman in her fifties. Her voice sounds fine to me, though I do recall hearing that the recorded high notes were by Elizabeth Schwarzkopf.
Why is Flagstad the most respected of all the Wagnerian sopranos? She captures the long phrase. Her interpretive eyes remain on the horizon. She can crescendo a phrase longer than you thought possible. In her voice Wagner grows into the giant we know. She lives in the realm of forever.