essy

Its very hard to rate guitarists like this. I mean, comparing Mark Knopfler and Kirk Hammett just doesn't sound right to me. Both are great but in different aspects. So, if someone makes different categories of great guitarists it will be much more accurate. Some of these categories can be -
1. Rock Pioneers
2. Metal Soloists
3. flamenco/classical
etc...

cornelius

hmm, this is a hard one... depends on what you mean...
either you specify the genre ( like essy mentioned) or on what level you mean " greratest ( emotional/ technical/...)

Sibeling

Steve Vai should also be on the list.