[u]Underrated[/u] and hardly ever mentioned among the greatest guitarists of all-time:

Frank Zappa, not that many people talk about his guitar skills. Zappa as an artist is underrated and often forgotten by many.

Neil Young, his sound creations and feel for the music are great. He is not as technical as some of the known names like Vai, EVH,etc.

J.J. Cale. people seem to always talk about Clapton, but hardly anybody plays as smooth as JJ.

Jonny Greenwood (Radiohead), speaks for itself, just listen to OK Computer

Also a lot of Metal guitarists are underrated, for example guys from Mercyful Fate or Andy LaRoque from King Diamond

[u] [/u]