Being a bassist, and having jammed with really excellent guitarists, it's really not too hard for me to name some guitarists I feel are underrated and people I wished I could meet and jam with at any given time.
I have a pretty large musical palate, so I'll try to break them down into general categories:
Metal:
-Jeff Hanneman (Slayer- extremely fast, but his solo work is pretty clever)
-Chuck Sculdiner(RIP) (Deah- yet, another fast guitarist. Very underrated, even though he may be credited as the Father of death metal)
-John Petrucci (Dream Theater- very technical)
-James Hetfield (Metallica- outshined by Kirk Hammet to the casual listener)
-Tony Iommi (Black Sabbath- need I say more?)
-Danny Spitz/Scott Ian (Anthrax- they worked well as a duo and both had their own unique styles)
-Josh Homme (Kyuss- anyone heard these guys? Awesome!)
-Al Jourgensen/Mike Scaccia (Ministry- what key are they in?)
-Jay Abbene/Terry Carter (Wratchild America- hell, the whole band is underrated and problem unknown to most)
Rock/Pop/Jazz/Punk:
-Brian May (Queen- elegant meets fierce?)
-Jeff Buckley (RIP)
-Alex Lifeson (Rush- we all know Geddy and Neal Peart, why is it we never hear about this guy?)
-Bob Mould (Husker Du\Sugar- the man's a legend)
-Prince
-Andy Summers (Police- great guitarist... I've met several people who never knew Sting played bass!?!)
-George Harrison (You may have heard some of his stuff)
-J Mascis (Dinosaur JR)
-Wayne Kramer (MC5- he's in a punk band on the Epitaph label these days)
-Brian Setzer (Stray Cats\Brian Setzer Orchestra- one of the coolest around. Have you seen his signature guitars?)
-Steve Howe (Asia\Yes)
-Greg Lake (Emerson, Lake and Palmer)
-George Barnes (RIP) (outstanding jazz musician)
-Johnny Ramone (Ramones- never hear enough about this guy)
-East Bay Ray (Dead Kennedys)
I know there are plenty more underrated guitarists out there... I just didn't want to take up the whole page.
Peace,
Randall