Well, as mj says, it's all a matter of what you enjoy listening to, unless you want to "study" the medium and if so there is a lot of music here representative of the period.
My favorite entry in the series is not in this box that I can tell, and is possibly more technically gospel than blues: "The Complete Blind Willie Johnson."
http://www.amazon.co.uk/Complete-Blind-Willie-Johnson/dp/B0000028QB/ref=sr_1_1?ie=UTF8&qid=1421882251&sr=8-1&keywords=The+Complete+Blind+Willie+Johnson
These 30 sides really get to me. I can recommend them. . .though this one 2 cd release is about half the cost of this set of 20.