I have yet to see anyone list Prince as an underrated guitarist, so I'll throw his name out there. Neil Giraldo is overshadowed by his wife's fame (Pat Benatar), but is a solid guitarist in my opinon. And here's one probably no one has thought of: Andy Griffith. Ever hear him play fingerstyle guitar on his show? Not bad! Also Roy Clark of "Hee Haw" fame.
Overrated for my money is Lenny Kravitz. He appears to be trying to be someone he isn't. Eric Clapton, although very sound technically, isn't someone I find particualrly innovative either. Regaring Jimi Hendrix I'm on the fence. To me, he's the KISS of guitarists. How much substance remains after all the smoke and mirrors (pedals, performing style, dress) is open to interpretation.