I've studied master guitarists for several decades by reading books and listening to recordings, but until YouTube I haven't had an opportunity to actually watch them play except on the occasions when I've been able to see a concert. As a historical archive, YouTube is a rich resource, something that has never before been so readily available to students of the guitar. These are good clips too: you can see close-up views of technique and fingerings. And using YouTube is simple. Yet without actual names in mind finding the good stuff can be a challenge. There are many clips available, but the site is inundated with beginners posting clips of themselves learning how to play popular songs. Though with their names at hand, the masters are just a click away. To help get you started here are some of my suggestions for a YouTube Intro to Masters of Guitar.
The obvious place to begin is with Andrés Segovia who devoted his life to the classical guitar. You'll find clips covering several decades of his career, including those of him playing into his nineties. Even if classical guitar isn't your thing, it's instructive to see the way he holds his instrument, his technique and the ease with which he plays difficult music. The fact that Segovia had such a long career is a testament in itself of the value of classical technique in protecting your tendons. Look closely at his form and technique. If you're familiar with the pieces he's playing, take a close look at his fingerings and his right hand technique.
Another classical powerhouse is Julian Bream. In the YouTube clips you can see subtle differences in the fingering and technique between Bream and Segovia. Many people actually preferred Bream's playing to Segovia's. As a youth I used to go to their concerts with a pair of binoculars to see how they fingered particular passages that I was struggling with. I have learned so much about approaching the guitar from watching these two and I would recommend their recordings to any guitarist. They both took very difficult music, much of it written for other instruments, and played it with great facility on the guitar.
Segovia had influence even beyond the classical field. He spent years playing concerts, making records and transcribing music for the guitar and he managed to make the instrument "respectable." He became the guy to go to if you wanted to be taken seriously as a guitarist. There are a lot of stories about these meetings including the one between Segovia and Chet Atkins. Apparently everything went fine until Segovia asked Chet what kind of music he usually played and he replied "country music." After that Segovia wanted nothing to do with him. If you look at the earlier clips of Chet Atkins and the later ones though, you will notice he holds the neck of the guitar higher (more like Segovia) after that famous meeting. Chet Atkins often said that Segovia had a big influence on him and you can see it in the clips.
An interesting contrast to Segovia is through his countryman Manitas de Plata, the father and uncle of members of the Gipsy Kings and a master of flamenco guitar. The difference between how Manitas de Plata strikes a string and Segovia is huge. Both are Spaniards, but they are masters of different kinds of music, classical and flamenco. Flamenco is improvised around various rules, whereas classical guitar uses sheet music that is carefully arranged and fingered. Even though both men are using nylon string guitars, they are speaking totally different languages. Their techniques have things in common, rest-strokes and free-strokes, but their approaches are different. Although both are very powerful players, Segovia produces a very contained sound, whereas Manitas is loose and brassy. Also Manitas has a wide array of strumming styles that someone of Segovia's ilk would never use.
Steering away from Segovia and the Spanish and classical influences altogether, a good look at the history of guitar mastery can be found through several early 20th century Americans: Lonnie Johnson, Eddie Lang and Lead Belly who represented blues, jazz and folk guitarists respectively. Their techniques and the styles of music they played are very different from that of the Spanish guitar of the time. All had recording careers in the 1920s.
Guitarists like Jimi Hendrix, Joe Satriani, Joe Pass, Kenny Burrell, Django Reinhardt and Steve Vai are other good clips to watch. For me they're less interesting as far as studying technique, but they should be listened to for the music they produced and their impact on guitar playing in the 20th century.
In my opinion, a particularly interesting guitarist to study is Michael Hedges, who is a world unto himself. Musicians have been experimenting with tapping technique on the guitar for many years, but he elevated it to an art form. After he burst on the scene in the 1980s many guitarists of different styles were heavily influenced by him. It seemed like everyone wanted to play like Michael Hedges. More than just being technically frightening, his musicianship was deep and profoundly rooted in classical guitar and jazz.
Hedges' influence can still be seen among contemporary percussive guitarists such as Preston Reed and Kaki King. They're essentially playing in a Hedges style. Comparing their clips is a good study in the evolution of musicianship. Hedges is a total master: he plays extremely difficult music without making a mistake, cleanly, precisely, and with great passion. Preston Reed plays it very well. And the truth of Kaki King is that she plays the same sort of music in a simpler form, but her live performances are full of errors. If you compare the three, you get a good sense of levels of difficulty and levels of mastery. In Kaki King's performances you hear buzzes and far fewer notes, some of which are dead. Preston Reed is solid and has been at it a long time. He's very good at what he does, but doesn't have the deep foundation in music that you hear in Hedges. Hedges is playing a whole different level, a lot of complex, multi-dimensional music seemingly effortlessly.
When I was younger it was fashionable to be self-taught on the guitar, but it's much more efficient to learn from people who've paved a way. That's how the study of a discipline evolves. YouTube offers you an opportunity to see exactly how some of the most influential guitar players addressed their instruments, and I highly recommend taking advantage of this great resource.