Hendrix absolutely revolutionized the art of guitar playing.
He completely changed how people thought about and used guitars.
Define greatness however you want, but Hendrix essentially *created* from scratch what we consider the modern guitar sound. Petrucci, Malmsteen, Van Halen, Clapton, etc, were all highly influential and are extremely proficient guitarists, but even Petrucci and Malmsteen will almost certainly tell you that Hendrix is *the* most important figure in the history of modern guitar technique.