OVERRATED
-Billie Joe Armstrong (okay, he writes great punk riffs, but "Greatest Rock Guitarist of 2005?" Jeez...)
-Zakk Wylde (like Malmsteen - fast, but repetitive, showy and forgettable)
-Yngwie Malmsteen (yadda yadda yadda)
-John Petrucci (I do like this guy - his bluesier stuff is great - but he does overdo things a bit)
-John Mayer (he just hasn't got soul! :( )
-Tony Iommi (great riff writer, definitely. The greatest? I don't think so)

UNDERRATED
-Andres Segovia (who's your daddy? :D )
-Dave Gilmour (this man is a genius! He does not get enough credit)
-Chris deGarmo (yeah, a personal favourite, but I seriously think he's got much more ability than many people give him credit for - he's a great songwriter, and his solos are just unforgettable)
-Darren Middleton & Ian Haug of Powderfinger (A pair of talented guitarists with incredible versatility. Listen to "Odyssey No. 5" - they touch on everything from blues to pop, psychedelia to metal)
-Matt Bachand (Another personal favourite :p , but he's overshadowed by Jon Donais simply because he [Donais] is the lead guitarist. Deserves a lot more credit for Shadows Fall's sound)
and...
KIRK HAMMETT (Go ahead and flame me, but I'm not joking. People slag him for being trashy and commercial - and maybe he is - but let's face it: he's one of the men who inspired a generation of rock guitarists to pick up an axe. Granted, his solos on "Kill 'em All" and "Ride the Lightning" are pure cheese, but his later work is much more controlled and tasteful)

Sorry for rambling on, guys. You can rip shreds off me now lol