I would add Tampa Red for slide guitar.
Since just about all the originators have been named here I'll mention a few contemorary acoustic bluesmen. There's Taj Mahal, of course, and Ry Cooder's early albums had some excellent blues guitar and mandolin work. Corey Harris and Guy Davis are two young performers who stay pretty close to tradition. A particluar favorite of mine is "Sidways in Paradise" by John Mooney and Jimmy Thackery, two normally electric blues players having some fun with acoustic guitar, mandoln and mando-cello.