Here are three guitarists I've found are underrated:
1. John Fogerty. Yeah, the early Creedence stuff is fairly simple, but he's really turned on the juice the last couple of years. He's actually studied and practiced since the breakup, so he's a much better guitarist now than he ever was in CCR.
2. Phil Solem, of the Rembrandts. While they're best known for the "Friends" theme, Solem handled some incredible axe duties on such tracks as "Follow You Down" and "Rolling Down the Hill."
3. Glenn Tilbrook, of Squeeze. OK, so this man can do everything, and thirty years on still has that sweet voice. However, his electric guitar leads reach virtuoso level, and his solo acoustic shows are a real treat as he condenses all aspects of Squeeze songs down to one acoustic guitar.
Honorable mention -- Prince. He's weird, he's perverted, but he smokes on guitar. He played "While My Guitar Gently Weeps" at the R&RHOF last year and blew Eric Clapton out of the water.
Lando
Add Warren Haynes - most underrated, hardest working guitarman in the bidness.