Thinking of current players who are under 50, I’ll go with Neal Casal currently of the Chris Robinson Brotherhood, and formerly with Ryan Adams’ Cardinals (he came on in 2005). He isn’t flashy, he’ll be staying within the constructs of the song, and then as many Brotherhood songs allow him to do he’ll take off into spaceland. I greatly enjoy hearing we he goes in those moments, and he’s very good at seamlessly passing over to the keyboards where sometimes I’m not exactly sure when the keyboards taking over, which sounds idiotic because they’re such different instruments. He’s not just a “spacey” player as he also has B-bender that can make him sound somewhat like Clarence White. The band draws covers from a wide range of artists (Byrds, Steppenwolf, T.Rex, VU, Hank Ballard, too many to list) and so it’s interesting to hear his approach to songs that you know well. He also was behind the “between the sets” music for the Fare Thee Well shows, which perfectly encapsulates his ability to show an influence but not carbon copy it. Oh, and he's far better than my attempt at describing his abilities.
Michael Hedges. Who I saw live in Pittsburgh. In the front row. Andy Summers doesn't get enough credit, either. I saw him live in Florida. In the front row. When the Police were in town decades ago. These were actually the only shows I've ever been in the front row for. Except for the time I made my way up to the front of the stage at a Billy Joel concert sometime around 1979 or 1980.
Tim Renwick is my choice. Before Peter White joined Al Stewart's band (and went on to have his own fame after a long time with Stewart) Renwick handled most of the leads. This includes the leads on "Year of the Cat," but he really stretches out on two songs from Modern Times: the title track and especially "The Dark and Rolling Sea." His playing is very lyrical and tasteful—it's surprising he rarely if ever made guitar hero lists. He's a respected player who has toured with both Gilmour and Waters solo as well as Eric Clapton.
(the late) Sam Andrew of Big Brother; Gene Cornish of The Rascals; Carl Giammarese of The Buckinghams; Michael Allsup of Three Dog Night; (the late) Don Rich of Buck Owens' Buckaroos; (the late) Terry Kath of Chicago; Mike Nesmith.
I hate to be that guy, but Michael Hedges and Andy Summers are in no way underrated. They are both very well known and extremely highly regarded.
Shelagh McDonald Paul Page who I'm not sure is still playing Jim Duckworth who I wish there was a lot more easily available by Oscar Moore King Cole trio geezer quite stunning. Roger Saunders who may not be on officially released disc yet. Allmen Joy were a very undersung San Francisco psych band who are better known as a supporting act on a few period posters. Darby Slick love his raga guitar style but again wish there wasa lot more of him recorded from the time. He took off to India at the time that Grace joined teh Airplane so there isn't more of him from the time. Charlie Pritchard wish somebody would either reissue Conqueroo From the Vulcan gas Co or make a new compilation. It's been like 30+ years since that came out. Guy Kyser possibly mainly in tandem with Roger Kunkel as the twin guitar throust of Thin White Rope. About the best twin guitar thing since Television. Though I may be missing more recent competition.
I don't think he's under rated as everyone I know and EVERY musician I have ever met think Roy was awesome. Roy in 1975
Mark Sandman (RIP) - between his 2-string slide and unitar bass guitars, no one ever did more with less.
Coco Montoya and Joey Delgado - both great guitarists who enough people haven't heard. And Mike Zito too!
I think many of you are confusing under-rated and not well known or popular which are 2 entirely different things. I don't know anyone who has listened to early Chicago ever say Kath wasn't anything but a great guitarist. The same is true for Jim McCarty who I saw perform with Mitch Ryder and Buddy Miles Express as anything less than great. Vince Gill is a highly rated guitarist as is Nils.
Don't worry. You're not "that guy". And someone can be "very well known" and "extremely highly regarded"...and still be underrated.