Well, how does one rate the greatest rock guitarist of all time? I mean I, obviously, see Slash as one of the most iconic-looking guitarists of all time (which is mainly due to the top-hat image and creating the recognizable guitar riffs of those hit songs from AFD) along with Hendrix, Eddie Van Halen, Jimmy Page, etc. but does that make him the greatest? It's hard to say, honestly. I will say he is a solid guitar player and songwriter, instrumentally, and that he's a great live player (and I can say that seeing his solo band live, last year).
I frequent the GNR forums, at times, and some people there, obviously, idolize him as their guitar hero and love what he's doing in his career now in terms of working on his solo band with Myles Kennedy and the Conspirators. In terms of the discussion of "greatest" though, I think they just try to stray away on that subject since they know it just creates a lot of debate that does not really move forward and starts a negative vibe and turns it into an Axl vs Slash or New GNR vs Old GNR debates that goes nowhere.