I have been fortunate to see some great blues guitarists live. I saw SRV three times and he was a true force of nature when playing live. As good as his albums are, they do not do justice to his power as a player. I saw Buddy Guy in a small club in Northhampton MA, The Iron Horse about a week after SRV died, and it was a very emotional experience. Luther "Guitar Junior" Johnson showed up and sat in as well. I bought several rounds of shots for them during the intermission and got Buddy's autograph on a piece of cardboard from a beer case! Buddyis the real deal. I have seen B.B. King several times and was also fortunate enough to see Albert King when he was still alive. I have also seen Eric Clapton, Otis Rush, Robert Cray, Johnny Winter, and many others from when I lived in Chicago.
I would not want to pick one as the best, there are many great blues guitarists, but they are all different and all with their particular strangths. If I was forced into a corner, I would have to go with SRV, but on other days I might go with Buddy Guy. T-Bone Walker is also one of my favorites