I saw Keaggy sometime in the 90’s and he blew me away. It’s even more impressive knowing that he is considered an expert “finger style” guitarist (according to Wikipdia) but he is missing half his middle finger on his right hand.
In terms of pure skill and ability to push the envelope, who are the greatest guitarist of all time?
Just because no one has mentioned them yet
Steve Cropper
Link Wray
Dick Dale
Eddie Hazel
Doc Watson was in a league by himself. Awesome talent. I need to get Legacy on my mp3 player ASAP.
For all his clowning, Roy Clark does a hell of a job.
Whoops!
OK, here’s one I’m pretty sure nobody mentioned: Sabicas, generally considered the greatest flamenco guitarist of the 20th century.
Oh yeah, and Merle Travis… how many guitarists have a style named after them?
Skill and innovation is going to be a different from my favorite guitarists list. I love Petrucci, but he doesn’t push the envelope (in his genre) as much as some of the guys below. (Yes, he pushes it more than the vast majority of rock/blues guitarists, but that’s largely a part of a genre that was established ahead of him.) Likewise, I enjoy Satriani and Vai, but I don’t see them venturing off the trodden path.
The already mentioned Robert Fripp. The first King Crimson album is amazing. He’s forged new styles and pulled other styles into rock.
Jimi Hendrix. I don’t care as much for his music as others do, but I can’t deny skill. He seems like the most innovative of the blues-based guitarists of the era. I prefer Clapton, but Jimi was way farther outside the box.
Tom Morello. He’s not as skilled on the fretboard as Fripp or Hendrix (or at least not as showy about it), but the way he plays with the role and sound of a guitar is fun an innovative.
Frank Zappa, though I think he was far more innovative as a band leader and composer than as a guitarist. The Shut Up ‘n’ Play Yer Guitar series is an interesting enough listen for his guitar work that his other strengths pull him onto my list.
As much as he’s knocked at the top of this thread, Al di Meola deserves mention for pushing boundaries and skill. I don’t care for jazz, and I’m no fan of his music, but there’s no denying his skill or ability to push the envelope.
Leo Kottke amazing fingerstyle and 12-string work. Heck, have another one.
I’m going to show Larry Carlton a bit of love. FYI, he is the guy that laid down most of the guitar tracks for Steely Dan way back when.
As always I have to go with Steve Morse above all others.
I’m very glad I was able to see Michael Hedges once before he died. On the acoustic side I think he pushed the envelope the most.
Randy Rhoads. 'Nuff said.
Michael Hedges
Richard Thompson
Hendrix
Leo Kottke
I’m not a guitarist, but I’d also add Robert Cray and Lightning Hopkins. The other 2 on my list, already mentioned, Santana and Mark knopfler.
Ry Cooder
Adrian Belew
Where Dick Dale is the ultimate shredder of guitar instrumentals, Paul Johnson is the ultimate technician of that genre. (Something’s seriously wrong with the world when the composer of “Mr. Moto” doesn’t even have his own wikipedia page.)
All these lists… I always thought “guitarist” was singular…? 
Johnny Marr
Steve Stevens
Mel Brown
Johnny Thunders
Guitar Slim
John McLaughlin
Eddie Van Halen
David Pritchard
All the wizards from Thin Lizzy
I’ve enjoyed this thread a lot. Loads of folk I didn’t know about, but now do. Can’t say I’m a huge fan of the more technical (?) stuff, but whatever, it’s just an opinion. Richard Thompson is ace, and I didn’t know I knew some of his stuff, he’s also warm and funny:
Richard Thompson - Oops I Did It Again - YouTube (It’s a Britney song)
Kevin Shields must be mentioned in terms of what is possible to do with guitars