For many people, Eric Clapton for one, Robert Johnson was not only the King of Delta Blues but the finest blues man that every lived. It helped a lot that he died at the age of 26, so he's got that old romantic appeal, and also that there is a legend attached to his name - the Crossroads Myth.
The story goes that Johnson met the Devil at a Crossroads, who gave him the skill to play the guitar like nobody else could. At that time he was following Son House around the juke joints where he regularly played and he was pretty hopeless at guitar. Johnson disappeared and when he was seen again some months later, he could play guitar like the Devil himself, hence the legend.
Myself, I don't subscribe to either point of view. I don't think he was given supernatural ability, or that he was the best blues guitar player of all time. My guess is that he was driven for sure, and was like a sponge soaking up the musical ideas of everyone around him, but it must have taken an enormous amount of work and time to perfect his skills. However, it's true that his delivery was incredible, the vocals weaving in and out of his guitar picking, seemingly asynchronous to the rhythm.
Actually, Johnson's technique was quite limited musically. Someone like Reverend Gary Davis had far superior skills in terms of picking styles and breath of material at his disposal. He could play anything from delta blues, through gospel to ragtime and was equally at home with any of them. Davis spent year on the streets of Harlem singing about the Lord after renouncing the blues when he became an ordained church minister.
Big Bill Broonzy was another master of the genre who could turn his hand to Tin Pan Alley, folk or swing and was a huge star in Chicago during the 30s and 40s after arriving in the city from the South as a young man. He was re-discovered in the 50s when a promoter was looking for Robert Johnson to perform at Carnegie Hall in New York. Learning that Johnson was dead, Broonzy got the job instead and stunned the appreciative audience. From that time he toured Europe and enjoyed a resurgence playing the old acoustic blues music of earlier years.
Many artists could claim the title. Not only giants like Lightnin' Hopkins and Muddy Waters, but lesser known figures such as Scrapper Blackwell, Pink Anderson and Floyd Council.