It would be Anthology of American Folk Music edited by Harry Smith. I couldn't imagine a better introduction for someone interested in the real thing.
Also:

Doc Watson On Stage on Vanguard. A 2 LP set now on disc which contains some of the best country blues, old time music, flatpicking, fingerpicking, a banjo tune, Travis picking and ballad singing that you're ever likely to hear.

No More Forever by Dick Gaughan (I'm not sure that it's available on disc but his anthology Prentice Piece might do, if not.)

A Bill Monroe anthology that contains some of the early stuff with Charlie Monroe as well as the Bluegrass Boys. Don't remember what it's called but I've got a cassette of it which kicks ass.

Joan Baez on Vanguard 1 and 2.

For Irish music the first Planxty LP, the first Bothy Band and Andy Irvine and Paul Brady as well as Solas, their first one, so named.