Phil Keaggy - should be in the top five - don't believe me? Check out Agora (The Marketplace) , or his song Doin' Nothing . This guy was phenomenal. Even in those songs, he puts a tremendous range of technique, and lot of it is just impromptu.
Joe Satriani - the teacher of Kirk Hammet. Why isn't he there?
Steve Vai - crazy guitar technique. Goes a little overboard on that, but he's way above 99% of guitarists.
Buckethead - this guy has crazy sounds, and puts out a huge amount of material. If you want to learn new and different sounds for guitar, check this dude out.
