The best UK collection is probably the Greig-Duncan Folk Song Collection, but it's 8 volumes. As for one-volume works, I'd say Kennedy is best.

The best Irish collection may be Sam Henry's Songs of the People, but I find it a bit difficult to use - kind of a matter of "too much of a good thing."

The best U.S. Collections are Randolph's Ozark Folksongs (4 volumes); and the Frank C. Brown Collection of North Carolina Folklore (7 volumes). For a 1-volume U.S. work, I suppose I'd reluctantly suggest The Folk Songs of North America, but alan Lomax - although several of the state collections are better (Belden's Missouri book and Brewster's Indiana collection, for example).

Our Basic Folk Library PermaThread will give you lots more information.

-Joe Offer-