Jimi Hendrix is the greatest for his ability to just flat out play guitar.  While he is thought of as a lead guitarist, his rhythm playing is in a league of its own. Also impressive is his ability to perform across any style of music, whether rock, blues, jazz, gospel, psychedelic, etc.  Many great guitarists perfect what they do best, such as Jimmy Page's knack for the blues and hard rock, but it is rare to be outstanding within so many different musical styles. And let's not overlook his ability to compose songs.