It's damn silly that Joan Baez didn't occur to any of them, but it simply indicates that someone else occurred to them first, that's all. In other words, there's someone else they normally think about more often than they do about her. Who people think about is more a result of the success of present day mass marketing most of the time than it is of the comparative abilities of performers.

You don't like the list? Well, just make up your own list. ;-) That's what I usually do.

My quick list of my 10 overall favorite singers (not necessarily the "best", just my personal favorites) would be:

Bob Dylan
Joan Baez
Buffy Sainte-Marie
Jackson Browne
Mary Chapin Carpenter
Al Stewart
Gordon Lightfoot
Mark Knopfler
Leonard Cohen
Ian Tyson

Who's "the best singer" among them? That's impossible to say, and I wouldn't even bother trying to figure it out.