Call me crazy, but I would mention Prince as an under-appreciated guitarist. He's well-known for his "spicy" lyrical content and his grooves, but I don't think he gets many kudos for his guitar playing--and that dude can really play. Saw him on the Rock 'n' Roll Hall of Fame inductions a couple years ago playing "While My Guitar Gently Weeps" and it looked like the other folks on stage with him (Tom Petty and others) were shocked that he could play like that.

Someone mentioned Elliot Easton of The Cars earlier. That guy might be the all-time most under-appreciated guitarist. His solos on "Tonight She Comes" and "Shake it Up" are outstanding!!!

Eric