Eddie Van Halen, no doubt great, but not as legendary as some of the others mentioned.
Tony Iommi and Black Sabbath has had, arguably, the largest influence on the rock/metal sound than any other guitarist/band.
Now I won't say Tony Iommi was a brilliant technical guitarist, like Eddie Van Halen, but his tone and riff writings are still influencing rock and metal today.
As great as Eddie is/was technically, and he's awesome no doubt, he was absolutely destroyed by the likes of Paul Gilbert, Yngwie Malmsteen, Steve Vai, <insert blazing fast ultra technical guitarist here>. The issue there was most of those ultra technical guitarists lacked the style of Eddie, and lacked the melodic structure and power of the name of Van Halen,
Keep in mind that Tony Iommi (Black Sabbath), Jimmy Hendrix, Tom Shultz (Boston), and Brian May (Queen) can also be grouped with Eddie in the "Created a unique sound that influence a genre" class of guitarists. In fact Tom Shultz, an MIT grad, created the FUZZ effect so popular and prominent in Boston music. They just lacked the follow-up ability that Van Halen had. This had more to do with timing than anything else though. Van Halen hit at a time when "party rock" exploded and ACDC was up there on top with the other "party rock" greats.
I can't even believe someone has chose to include that coward suiciding poor excuse for a human, Kurt Cobain, in this thread. Come on, get real. Nirvana was innovative in launching the Seattle scene, and creating a new, spin off, on rock/metal (the grunge movement). But, technically, and in the context of this thread, they, not Cobain or anyone else in that band, holds a candle to the likes of some of those listed here. He was average with cool new sound, end of story.
As I wrote this a post was made on Randy Rhodes. I actually thought about him for this thread days ago. Upon realizing I didn't mention him I felt like I betrayed him and this thread. Randy Rhodes was a genius. So amazing!
To me, Jeff Hanneman and Kerry King (Slayer) are two of the greatest guitarists due to their technical rhythm abilities. No, they aren't blazing fast and technical lead guitarists, but their rhythm creations and style, equally as fastly played as most leads, is second to none.
In all honest, this subject is too broad and too, well, subjective. There's a world of guitarists out there that effect each and every one of us differently. There is no way to develop a criteria for "BEST GUITARIST EVER." There are so many that are so great in so many different ways.
What is a "greatest guitarist," how and why?
Let's take the classical genre and ask, "Why are we not considering them as the greatest guitarists EVER?" In the old days of solos, duets, quartets, sonatas, concertos, symphonies, and opera, where being a musician was a lifetime, 24/7 dedication, gaining you lucrative employment under a king, if you were good enough, being a musician was about more than gathering a bunch of marginally talented dudes together in your garage and banging out tunes while you, your mom, dog, and neighbors went def. These musicians played by the sheet, and wrote by the sheet. Their pieces were under the utmost scrutiny, potentially costing your life if undesirable. You wrote, you played, and you taught. That's what you did. That's all you did. You were a master of your trade, or you worked a different trade. Popularity was never by accident, or beer induced. Popularity was based on talent and desire.
Some classical guitar pieces are the most intricate, melodic, and beautiful guitar works ever made. They are not just appealing notes played together. They are master pieces eliciting journeys in the mind. Multi-dimensional and unfathomably deep. Soothing and peaceful, as well as, vigorous and exciting. Classical music, in general, is music for all eternity. We (human kind) may never again live in a time where it is commonplace to expect such dedication and perfection in musical works.
To bring up this thread, and not consider the "masters" of guitar from an era long past, gives little credibility to your knowledge of music, and the guitar specifically. The guitar was not invented by Jimmy Hendrix or Jimmy Page back in the 60's. They simply derived their technique & styles from centuries of influence cascading down from each guitarist before them.
Broaden your horizons...Check them out...
Paco De Lucia
Al DiMeola
John McLaughlin
http://www.youtube.com/watch?v=EfTzojPY_I8John Williams...I can play this on guitar, though not this well:
http://www.youtube.com/watch?v=oEfFbuT3I6ADo you laugh at this list, like I do:
http://www.rollingstone.com/news/story/5937559/the_100_greatest_guitarists_of_all_timeCobain-12, Eddie Van Halen-70...wtf...please!