Most guitarists who I would consider overrated I dont listen to much of their stuff so I wouldnt really be able to say.

Underrated though:

Marc Bolan (T Rex) - Great rhythm player, so of his ideas were very innovative. Pity he died so young.

George Harrison (Beatles) - The most underrated guitarist, musician and songwriter ever. Proof is that Frank Sinatra(phoney bastid that he is) used to introduce his version of George's Beatles song "Something" as "his favourite Lennon/McCartney song ever.

Keith Richards (Rolling Stones) - Something alot of guitarists dont understand is his genius. Funnily enough, his biggest fans and biggest critics are all guitarists - the critics usually made up of lead-minded players(but not all mind you)

Robby Krieger (Doors) - Great classically-styled guitarist guitarist. Very underrated but his solo from "Five To One" has been ripped off by KISS and Pearl Jam.

and
Noel Gallagher (Oasis) - Why am I the only one who thinks he is a good guitarist? He can solo, thats for sure. And he can sing and play, sing and solo at the same time(which we all know can be really tricky). which is better than alot of people can do.