If you are truly a lover of the blues than you know that the artist is just a conduit to the music. So their is no best.
Their may be artist that do this better "for you" than others but that does not make them the best.
Clapton is no doubt a talented guitar player. However, as it has been mentioned he is not really a blues guitarist.
Listen to Son House, R.L. Burnside, Albert King, Buddy Guy, Mississippi Fred McDowell, Lightin Hopkins, T-bone Walker, Elmore James, Hubert Sumlin, Muddy Waters, Blind Willie Johnson, Ship James, Blind Willie McTell, Big Bill Broonzy, Mance Lipscomb, Mississippi John Hurt, Freddie King, Jimmy Dawkins, Albert Collins, Earl Hooker, J.B. Hutto, Luther Allison, Guitar Gabriel, Otis Rush, Junior Kimbrough, and John Lee Hooker to name a few. After listening to them you will realize that Clapton does not play like this.
Does it mean that Clapton is less of a guitar player than those mentioned. Probably not. Is Clapton a better blues guitarist? Probably not.
To show you how opinionated guitarist can be Rolling Stone comprised there list of the best guitarist. I do not agree with this list or for that matter understand how you could rate them as they have different styles. Even so look where Clapton appears.