Like many other people who have posted here, I don't think there's a single "greatest" player of the blues. But if I had to choose, I'd personally say either Robert Johnson, Hubert Sumlin or Elmore James.
The ones named are great but I don't know if there is a "greatest of all times" player. Bobby Blue Bland had one during the 1970s, I never knew his name, who was as great as any I've ever heard. Johnny Taylor has had some mighty fine players also, so I don't think it's possible to say there is a "greatest of all times" blues guitarist.
These are always tough questions to answer.. I think Robert Johnson was one of the greatest because of his influence on other great blues guitarists. Definitely I would place BB King near the top. Another one that didn't have too much attention in the US, but was one of the greats is Luther Allison.
I listened to a lot of blues during my teen years, and was able to meet both BB King and Luther Allison when I worked for a non-profit organization that booked bands in the central Illinois area. I also had contact with the now very famous (wealthy) Irving Azoff who managed our bookings.