over: I've got to say that Clapton is a little overrated and John Mayer. John isn't that great a blues player and most of the time he is playing Clapton or S.R.V. licks.
Under: Albert Collins and the most powerful blues ever. he is so soulful and always has a great funky Rhythm Section.
also, Jerry Garcia, his pharsing is stunning and his triplet runs are completely unique
Dickey Betts, he has always been in Duane's shadow and he wrote some of the biggest Allman Brothers hits as well as playing some solos maybe better than Duane
As for Jimi I think he is underrated in the sense that he did so much for the electric guitar. We remember him for "Purple Haze" and "Fire" when his best stuff wasn't the hits. Listen to "A Merman I Should Turn To Be", "Who Knows" "Machine Gun", "Bold As Love", and "Voodoo Chile" (the bluesy version) then tell us he was overrated.
Also Jack White is a better guitarist then people give him credit for. I can't listen to Dead Weather or the Racontuers, but the White Stripes had some really great blues stuff.