Bearing in mind that everyone has a different opinion and that there is a lot of really great music in more obscure recordings, I would nominiate "Robert Johnson - The Complete Recordings" on Columbia, and "Muddy Waters -The Chess Box" on Chess Records. This covers Delta blues in the 30's and Chicago blues in the 50's; pretty hard to beat.

I personally like country blues with a litte rag to it so I would also recommmend anything by Blind Willie McTell or Blind Blake.