Forest Green is Vaughan Williams' arrangement of The Ploughboy's dream, and is by far the most common tune for this hymn not only in Scotland, but in England and most of the Commonwealth.

H. Walford Davies wrote two tunes for this hymn, Wengen and Christmas Carol, neither of which I know.

Most information here is from Wikipedia, although I did know where Forest Green came from (Vaughan Williams collected it in a village of than name in Surrey, which, paradoxically, is not the home of Forest Green Rovers who come from Gloucestershire).