"Ar" wants to know who they are. When I was young I studied guitar for seven years (multiple styles), so it's an area I've long had an interest in. I was never very good but I learned a lot about it. Here goes:
Classical: Segovia, Eduardo Fernandez. I enjoy the transcriptions of Yamashita and Larry Coryell's covers of Stravinsky, though he isn't usually considered a classical guitarist.
Jazz: Django Reinhardt, Joe Pass's Virtuoso album, Wes Montgomery live (no strings), and George van Eps. Charlie Christian deserves a mention. Today, Kurt Rosenwinkel, and the guy who plays for Trio Saudade. There are plenty of others, including Jim Hall and John McLaughlin.
John Fahey-Leo Kottke: They deserve their own category and indeed they dominate it. For Kottke try 6 and 12-String Guitar Music, and then his 1981 Guitar Music. For Fahey try the 1959-1977 Greatest Hits collection. This is some of my favorite music.
Electric blues: Muddy Waters, Robert Cray (live), Johnny Winter (live only). Amadou of Amadou and Miriam. The player from Orchestra Baobab. Does Lonnie Mack count here?
Acoustic blues: Reverand Gary Davis, Son House and many others. Jorma Kaukonen also. Bob Dylan is much underrated in this area. Can Richard Thompson go here? d'Gary, from Madagascar, is one of the greatest and most original guitarists that few people have heard of. Bola Sete too, from Brazil.
Bluegrass: Clarence White and also Doc Watson.
Rock: Jimmy Page, Brian May, Chuck Berry, Bo Diddley, of course Jimi Hendrix as #1. van Halen and his ilk never much impressed me.
Les Paul deserves mention but he straddles a few of these categories, as does Chet Atkins. Hawaiian guitar deserves its own post. Dick Dale. The Carnatic slide guitar players, including Bhattacharya. Roger McGuinn. The Zairean tradition, including Franco. Neil Young has his moments, as does Thurston Moore.
Eric Clapton was impressive for a while but overall I wish to be contrarian and leave him off. Who am I forgetting? Duane Allman?
In general guitar is an instrument which works relatively well on YouTube. Most of the names above can be found there.