Apart from some of those mentioned above, particularly the complete Robert Johnson, Mississippi John Hurt's 1928 session, the 2 Sonny Boy Williamsons (John Lee and Rice Miller), Son House and Charlie Patton, CDs that I would not be without would incude:

Bessie Smith 'Complete' (10CD)Sony
Howling Wolf box set(3CD)Chess/MCA
Muddy Waters box set (3CD)Chess/MCA
Skip James 'Greatest of the Delta Blues Singers' Biograph
Sleepy John Estes 'First Recordings' JSP
Lightning Hopkins 'Gold Star Sessions Vol 1&2' Arhoolie
Blind Willie McTell 'The Definitive' (2CD)Sony
Various 'White Country Blues' (2CD) Sony
Big Maceo 'King of Chicago Blues Piano' Arhoolie
Blind Lemon Jefferson 'Same' Ace
Johnny Shines 'Standing at the Crossroads' Testament
Little Walter 'The Essential' (2CD) Chess/MCA
Various 'Canned Heat Blues: Masters of Delta Blues' RCA Heritage
Blind Willie Johnson 'The Complete' (2CD) Sony
Hop Wilson 'Houston Ghetto Blues' Bullseye Blues
Elmore James 'Classic Early Recordings' (3CD) Ace
Cannon's Jug Stompers 'Complete Works' Yazoo
John Lee Hooker 'The Ultimate Collection' (2CD) Rhino
ZZ Hill 'Greatest Hits' Malaco
Eddie Hinton 'Letters from Mississippi' Zane
Robert Nighthawk 'Live on Maxwell Street' Rounder
Jimmy Reed 'Bright Lights, Big City' Charly
BB King 'Live at Cook County Jail' MCA

Cheers, Stewie.