The Columbia series (Roots and Blues) is still the best I think -- there is now a four volume box set. The first in the series (Vol. 1, that ends with Son House' Death Letter Blues, redone in the 1960's) is my favourite. If you want real old delta blues, Yazoo's Friends of Charley Patton is pretty good (and then there is Robert Johnson all on his lonesome).

yours, Peter T.