Three all-time greats that, for some reason, no one has mentioned so far (and that you don't hear much about):
1. Skip James--His "Devil Got My Woman" is the eerie song the lead character in the movie Ghost World falls in love with and listens to all night. His "I'm So Glad" was made into a rock hit in the 60's by Eric Clapton, but he hated that version, and so do I. (He did, however, appreciate the royalties.) Try to get the early recordings album. Yeah, it sounds a little scratchy and all, but he was at the height of his abilities when those sides were recorded. The later recordings, made after he was "rediscovered," are easier listening, and in some cases genuinely beautiful (his voice aged nicely), but you can tell his fingers aren't flying quite as fast as in the early days.
2. Rev. Gary Davis--He struggled most of his professional life with whether he could be a good Christian and a blues player at the same time. The ultimate answer is that he was both. Though he put out only one purely "secular" recording late in his life (at the exhortation of those who wanted his "pure" blues recordings saved for posterity), his religious albums exhibit some incredible guitar playing. He was blind, and he used an unusual technique which I don't appreciate (not being a guitar player myself). But I've got some DVDs of him playing and my sons are just astonished every time.
3. Robert Pete Williams--They say that he wasn't influenced by anyone, and that (unlike the other two) he himself didn't influence anyone else because he was so unique. I haven't heard anything like him, and I don't know how to explain him in musical terms, but the more I listen, the more I appreciate him. At the very least, I can say that his songs are intensely personal and original--no one else is likely to cover most of these. Make sure that in addition to his individual albums you hear the songs by him on "Angola Prisoner's Blues," an amazing field recording album. His song "Prisoner's Talking Blues" was supposedly improvised on the spot. For his guitar playing, though, "I'm Lonesome Blues" shows off his skills.
I've barely touched on these guys, so below are the links discussing them and their work on the allmusic.com website.