PRINCE!!! Definitely Prince. I would say he is not only an underrated guitarist but an underrated musician. That guy kills any instrument he picks up. I have nothing but the utmost respect for him.
John Mayer is another one. The melody line alone in "Neon" is one of the more difficult riffs I've ever tried learning (and I've been playing for over 21 years now). And to add insult to injury, the guy sings over the top of it too. Most people only know of his "pop songs", but check out his trio stuff. Absolutely amazing. The guy can play his butt off.
Definitely agree. I forgot how good a guitarist he was until I was reminded of the RRHOF jam of "While My Guitar Gently Weeps" with Tom Petty, Dahni Harrison (who is eerily a spitting image of his dad George), Prince, Jeff Lynne, etc.
http://www.youtube.com/watch?v=zByqXu6nGYA