As a guitarist and musician myself, I'm a little biased. When I think of "Greats," I lean heavily toward those that I have the hardest time emulating. Here's my cursory list of "virtuosos":
Steve Vai - you can't play it because he reinvented the guitar. Literally. Do some research.
Joe Satriani
Yngwie Malmsteen
Eddie Van Halen
Jimi Hendrix
These guys are awesome, too, just a little more replicable:
Clapton
BB King
Dave Matthews
Page
Harrison
Gilmour
Allen Collins (Skynyrd)
Petrucci (Dream Theater - don't care much for it, but he's ridiculous good)
Tom Morello (Rage)
There. That's a start!