Been reading back through all these old posts, and apart from the question, 'Where have most of these posters gone?' There's been a lot of good and erudite stuff been posted.
The problem is the question in the title, which is unanswerable with a single name, especially when it comes to performers, as personal tastes bias us on that one.
I think that the collectors are the people for which we have the most to thank, and they often did different things in different countries. So I've got to go for, Cecil Sharp, and Ralph Vaughan Williams for the English side of the pond, Francis Childs, and Marjorie Kennedy Fraser for Scottish stuff. Lomax and Moses Asch spring to mind for US stuff, but I don't know enough about that side to be dogmatic about it.
Interesting thread though.
Giok