One of my favorites that I don’t think was mentioned (I checked a couple times, but please forgive me if I missed it) is the late James Honeyman-Scott of the Pretenders.
Ritchie Blackmore
Tony Iommi
Dave Mustaine
Dave Navarro
Steve Stevens
There’s another guitarist that’s really good, Bill Massey of Oz Knozz. He just sounded good with his note selections.
Then there’s J Geils and John Fogerty.
These two are huge heroes in the jazz world and not at all underrated.
Danny Gatton used to be called the best guitarist you’ve never heard but now he is a legend, at least among guitarists.
Didn’t notice the dates on these posts. Well, at this point the only logical answer is Paul Atkinson.
I was a fan of Grand Funk when I was in high school but I always thought Mark Farner was a terrible guitarist.
Neil Young. Not gonna throw a thousand notes a minute at you but his solos are so melodic. In Cortez the Killer, the only chords are EM7, D and AM7 played in loop. The first and last chords only use 2 fingers. And who hasn’t been blown away by Like a Hurricane.
Willie Nelson. Great picking style for both country songs (check out Willie and Family Live) and classics like Stardust.
The most underrated rock guitarist is undoubtedly Paul McCartney, because no one thinks of him as a guitarist.
Consider just The Beatles songs where it’s generally acknowledged McCartney played lead guitar:
Ticket To Ride
The Night Before
Another Girl
Taxman
And Your Bird Can Sing (with George)
Sgt. Pepper 's Lonely Hearts Club Band (the song, not the entire album)
Good Morning, Good Morning
Back in the U.S.S.R.
Birthday
Helter Skelter
The End
The Beatles, without a doubt, Are the best band ever. (IMO)
Best answer in this resurrected thread. 
The Doors Robby Krieger. While overshadowed by front man Jim Morrison, all of the Doors music could Light MY Fire
Good answer. I hate the Doors, but Krieger really is excellent and by far my favorite part of that band, and he rarely gets the props he deserves. He is respected by a lot of guitarists, for sure, but he seems to fly under the radar for most people.
A couple of years ago, Rolling Stone put out a list of the “100 best guitarists of all time.” I was surprised to not see John Fogerty on it.
Sure, technically he’s certainly not among the all-time greats, but he has a unique, recognizable style. He’s created some of the most memorable licks in rock history. And, it’s intangible, but something about his playing just stirs me. The openings of Commotion, Up Around the Bend, Green River, and more just cut right through me somehow.
Just occured to me, listeing to Good Rockin’ Tonight by Elvis. Scotty Moore! Seminal guitar player.
From Wiki - The Rolling Stones’ lead guitarist Keith Richards has said of Moore,
When I heard “Heartbreak Hotel”, I knew what I wanted to do in life. It was as plain as day. All I wanted to do in the world was to be able to play and sound like that. Everyone else wanted to be Elvis, I wanted to be Scotty.
I think I just won this debate LOL
Oh and re Keith, he was mentioned earlier, thought of as mostly a ryhthm guitar player, but was also a pioneer in the early days of rock guitar. His bio is highly recommended.
From the book - Why did we stop at the 4-Dice restaurant in Fordyce, Arkansas, for lunch on Independance Day weekend? On any day? Despite everything I knew from ten years of driving through the Bible belt. Rolling Stones on the police menu across the United States. Every copper wanted to bust us by any means available, to get promoted and patriotically rid America of these little fairy Englishmen…The State Department had noted riots (true), civil disobediance (also true), illiicit sex (whatever that means) and violence across the United States. All the fault of us, mere minstrals…it had become, in the time of Nixon, a serious political matter