Okay, this is an impossible thread to try to respond. Are we going to include classical guitarists (Andres Segovia, Julian Bream, etc), flamenco/Spanish (Manitas de Plata), finger-pickers, slide players, etc, etc. I think for a start we should exclude anyone whose main instrument is electric guitar—not including amplified acoustic guitar. In one fell swope, we eliminate a bunch of those mentioned above.
Since there are still too many categories, I'll only mention blues(wo)men. The Reverend Gary Davis is a remarkable player. Mississippi John Hurt has a gentle, seemingly simple, melodious finger-picking style that is mindbogglingly complex upon serious listening (a perhaps apocryphal story is that an enthused student of Andres Segovia brought him a John Hurt recording, and Segovia wanted to know who the second guitarist was). Robert Johnson's flashing slide playing has inspired generations of musicians. Blind Willie McTell finger-picks 12 string guitar (!?!?) in a frighteningly virtuosic manner. Memphis Minnie was so good it is claimed she bested both Big Bill Broonzy and Tampa Red (both great guitar players) in a cutting contest. Muddy Waters claimed Son House was the greatest of all blues guitar players. Let's leave it there.
bjarmson