Anything by Josh White, but particularly his 25th Anniversary album with that wonderful talking/multi-song version of John Henry.
Bukka White would have to be in there somewhere, tho' I wouldn't want to choose a particular album.
A compilation CD called Streamline Special or Special Streamline, I can't remember which that has a bunch of great slide guitar players on it.
I'd second the nominations of Will The Circle Be Unbroken (NGDB) and the early Baez.
For great vocal arrangements and pure energy, Joe and Eddie. I have a couple of their albums on tape, but I don't remember the names of them.
And my all-time hero: Mance Lipscomb. Anything by him, but I've got about three hours of him live that I wouldn't take anything for.
I think if I were home I could probably come up with at least 100 that I COULDN'T LIVE WITHOUT, so I'll just sorta fade out of here ......
ddw