I don't/didn't like the voices of Bert Lloyd, Ewan MacColl, Shirley Collins, Burl Ives, Louis Killen or Peggy Seeger.

I did like, of the revival greats, Johhny Silvo (still alive) and Lal Waterson (sadly not). Martin Carthy had a great voice and it is still good but it is starting to age.

The individual voice in harmony bands are often not too great - take the Spinners, the Young Tradition, the Coppers - but the overall effect can be fine or better than fine. Peter Bellamy varied between inspired and unlistenable.

Ian Bruce is almost too good. Other current great voices: -
June Tabor
Norma Waterson (again, perhaps not what it once was)
Eliza Carthy
Peter Collins
Mike Nicholson
Tom Lewis
Both of Capella (but best together)
John Barden (on the soothing rather than fiery side)

I really do not approve of the breathy fey voices so much in fashion (both male and female). For me they are not great folk voices.