Somehow, I missed this thread 13 years ago. Maybe it's best I did.

But reading it now, I see so many 'favorite' singers put forward for a wide variety or reasons. It's hard to argue with some... like Jean Redpath & Edith Piaf, who did quite different things.

After over 50 years of listening to (mostly) folk music, I would not use the word 'perfect', except to refer to somone who's pitch is always right on...and Redpath comes close.

But I do tend to agree with several others that 'presentation' of a song in a believable, powerful way with emphasis ON the song rather than the singer is what impresses me the most.

The American Roscoe Holcomb (The High Lonesome Sound) was known to sing a moving ballad and then be emotionally unable to sing anything else for awhile.
And I seem to remember that Jean Redpath took quite awhile to even attempt "Sheath & Knife" after hearing Helen Schneyer sing it. (see the threads on Helen to get the point).
One of my own favorite performances is the recording of Davey Stewart singing "The Merchant's Son and the Beggar's Daughter"....his melodeon only made it stronger.

These days, I am quite taken with the huge variety that Danny Spooner does, all done well, but never making himself the point. Several years ago, when Danny was touring the US, I sat beside him in a pub which had notably mediocre acoustics due to a ceiling beam that divided the room in half. It was common for singers to walk to the center in order to be heard at all over the usual pub noises.
Danny & I were sitting at the far rear of the place, and when his turn came, I whispered that he might want to go to the center. He never replied, and never even stood up... then he opened his mouth and the place suddenly quieted down and people in the far front window turned to see who was overwhelming their conversation. THAT is singing (one type) and the song never sounded strained...just powerful.