Arguably one of the longest running and widespread ballads in the oral tradition
At the time of the Great Fire of London in the 1660s, Samuel Pepys commented in his diary ".but above all, my dear Mrs Knipp whom I sang; and in perfect pleasure I was to hear her sing, and especially her little Scotch song of Barbary Allen."
It runs into hundreds of versions and has been found all over the English-speaking world.

It's unique place in the tradition was summed up for me when she wrote about her collecting trip to Ireland in the early 1950s.
"When we asked for "the old songs" we were given 'Danny Boy', 'I'm 'My Dear Irish Home' and all the the sentimental stuff, but when we asked, "Do you know Barbara Allen", that's when the beautiful traditional songs came pouring out".
(paraphrase)

As far as I'm concerned, she reigns queen over her genre
Jim Carroll