"The Music of Bill Monroe, 1936-1994 (4 CD set)" MCAD4
"Muddy Waters, Folksinger" Chess
"The Essential Doc Watson" Vanguard
"Robert Johnson: The Complete Recordings" Sony/Columbia
"Lightnin' Hopkins" Smithsonian Folkways

And on and on.
I can't go to this desert island-- they won't let me take enough recordings. I would want to take everything everybody's already listed--even the ones I haven't heard (Celtic area), plus much more. Especially the Anthology of American Folk Music.