I have "discussed" this over the years countless times. Everybody has an opinion and non one ever changes someone's mind about this. But it is fun to chew on, so here goes.

Frist of all, I want to make a distinction. There are people with beautiful (I hate to use the word perfect about mere humans) voices AND there are people who are impeccable singers. Most people listen to the tonal quality of a person's voice and say, "there is a great singer". This is the same as confusing a guy with a great guitar for a great guitar player. I like singers; I hate it when someone with a perfectly good voice doesn't learn to use it.

I love Judy Collins' voice. She also sings well. Ditto for Joni Mitchell and the late Sandy Denny. Joan Baez, while having a good voice, IMHO never learned to sing.
Among the men I would rate Bruce Cockburn bery highly, also Geg Brown, Theodore Bikel, Frank Sinatra and Roy Orbison. No one puts as much emotion in a song as Roy Orbison. Some of the singers that I like to listen to a lot are Leroy Parnell, James House, Linda Ronstadt and Aaron Neville.

But without a doubt the singers I admire most are Frank Sinatra, Ruth Brown and Bob Dylan. Sinatra has it all. And few can interpret a song like he did. Ditto Ruth Brown. And Bob Dylan, although not blessed with a pleasant natural tonal quality, is unbelievably good with what he has. He is a great interpreter, particularly of the blues. If you need convincing on Bob, listen to his live recordings.