Depends on the music style.
Folks are mentioning Rock guitarists but dont forget the Blues ones. Some amazing ones out there.
Category:Blues guitarists - Wikipedia, the free encyclopedia
Not trying to knock anyone or not appreciating his talents but I always thought Eric Clapton was a bit over rated. I saw Stevie Ray Vaughn in concert too and he was amazing. Though not my personal favorite for music style but he blew me away how good he was. Knopfler is also up there in my opinion too and I like his style.
Though not the greatest but my personal favorites based on style is Stuart Adamson of Big Country/Skids and The Edge from U2. Another underrated guitarist is Andy Summers from the Police. I saw them over the summer and that concert was a billion times better then 1983. Brian Setzer is pretty slick too.