None of the guitarists that make me want to learn to play guitar seem to make any of the "greatest lists" (okay, maybe a few). So I feel that a lot of my favorites are underrated. Most of favorites are on my list because they make solid music.

Daniel Ash - Yes, he experimented a lot, but he was willing to push his boundaries.

Steve Kilbey, Marty Wilson-Piper, Peter Koppes - The second I heard their playing as The Church, I was hooked and "Under the Milky Way" is on my "Learn to Play" list.

Robert Smith - I feel that he wrote solid songs.

Nuno Bettencourt - He seems to make a lot of older lists as an "Honorable Mention", but I love his range of playing.

I totally agree with Dan809, Joe Walsh is a very underrated player. I've also had to explain to fellow guitarists that Paul McCartney was mostly a bassist (although he is one heck of a writer).

My overrated list would mostly include shredders (but I grew up listening to classic rock then switched to 80s "progressive/alternative" and "gothic" rock. (Not that modern "gothic" rock garbage.)