Some great suggestions so far, particularly anything that Willie Dixon played on, and any recording you can find of Son House. However, to make a less traditional suggestion, a good acoustic recording, with some blues aspects, is Eric Clapton Unplugged. I find this to be an especially involving recording, and I often use it when auditioning new equipment.
With regard to the other suggestions, I have many recordings by the above-referenced artists, but so many of them are not well recorded, mixed, or produced. Any suggestions on specific recordings that are worth acquiring? I was fairly impressed with what I heard on the PBS airing of Martin Scorcese's "The Blues", so I have high hopes for the derivative CDs and DVDs of that production.
Tom.