Hmm ... thought I might throw my two-happorth in.
Died for Love (preferably the version collected from Joseph Taylor by Lucy Broadwood and Percy Grainger in 1906 - great tune)
The Unquiet grave (got to have a revenant ballad of some sort)
Searching for Lambs (CJS thought it the most beautiful song - who could disagree?)
The Bonny Bunch of Roses
The Lover's Ghost or the Grey Cock (from Cecilia Costello - apart from the fact that I love the song, the words "the burning Thames I have to cross" always gets me intrigued - strong words which imprint themselves on the brain, but what were they originally before the folk process got involved ?)
The Seeds of Love (but not Mr England's version ... don't like the tune)
Bruton Town (a great example of a big murder ballad, with a story traceable back to at least Boccaccio. I remember having to study Keats' "Isabella or the Pot of Basil" at school - same story!)
All Things are Quite Silent (probably because it was one of the first folk songs I came across while around 9 years old, and because it reflects a historical practice, pressganging, which no longer existed when Vaughan Williams collected it - folk song as social history)
I give up - two more to go, and there so many that I'm fond of!! I'll stop there.
Incidentally, looking at some of the postings it's quite clear that some people's choices reflect where they come from! In my case it probably reflects the fact that I don't like chorus songs, and LOVE songs that are miserable or blood-thirsty!!
Just thought of one more - definitely need to have a broken token song, so how about Claudy Banks (I think it's already been mentioned, hasn't it?