I never understood rock guitar because it is based on riffs and improvisation and has little harmonic interest. Although I love Zappa's playing... but rock guitar isn't really "guitar". It's a melodic instrument that primarily focuses on jazz-style improvisation. Which is cool. But it doesn't capitalize on all the possibilities of the guitar. Looking at the topic from an objective point of view, flamenco is the most sophisticated form of guitar playing. Tell me if a lot of guitarists can do this:
http://www.youtube.com/watch?v=e8G0Abl6Dgg
It's a real shame that the video is out of sync, but oh well. Check out the riff starting at 0:50, or the one at 2:11. Most guitarists can't play that fast with a pick!!!
For jazz improvisation, NOBODY beats Joe Pass. The following is completely improvised:
http://www.youtube.com/watch?v=JFqiDcvRW2Y
As far as classical guitar, I like John Williams...
http://www.youtube.com/watch?v=wDAHl54V0CU
And my favorite guitarist of all time, Paco de Lucia. The actual tune starts at 0:56, it's the first track off of his "Luzia" album...
http://www.youtube.com/watch?v=WaJPx44L86I
