That is a tough call, depends on how you define the greatest. I've seen a few guitarists that aren't famous that would blow your mind. My buddy's guitar teacher Bruce Shockley is the best I've ever seen. He lived and played in Nashville for years, and was known as "the baddest guitar player in Nashville" by many.
As for big names, you could say Stevie Ray Vaughn, Jimi Hendrix, Eric Clapton, and the list goes on. Some really good guitarists currently are Brad Paisley, Keith Urban, Prince, and the slide guitar player from Union Station (Allison Krause's band). I've seen some classical guitar players that are amazing too. Picking a greatest of all time is pretty much impossible though.