It doesn't have to be rock guitarists, does it? My list is all guitarists whose work is recognizable at a few notes.
1) The Edge -- because he is a true original
2) Les Paul -- the inspiration for generations that followed
3) Chet Atkins -- one of the loveliest men to ever walk this earth, as well as a fantastic artist
4) Mark Knopfler -- a real musician's musician, able to play just about anything
5) Carlos Santana -- so sexy, and proof that music can excite and soothe
Honorable mentions have all been listed by other posters -- and Bonnie Raitt is a fine guitarist, herself!