Overrated Kurt Cobain I guess it's my prejudice against grunge music as a whole but I just don't understand why he's hailed as innovative and profound.
A lot of good underrated ones listed already, Ross the Boss, Vito Bratta, Vai, Satch, PG and Yngwie although within the guitar community I think the latter 4 get the respect they deserve.
Reb Beach of Winger has some great chops and lays down awesome rhythms I really like this song, it was on an instructional vid I just think it's a really cool jam. His 2 handed tapped string skipping is really killer.
YouTube - Reb Beach Cuts It Loose
Tony MacAlpine also a pianist, played keyboards/Guitar for Steve Vai for a while has some solo stuff which is fantastic shred along the lines of Yngwie. My favorite though is his album with Rudy Sarzo, Tomy Aldridge and Rob Rock M.A.R.S. Project Driver
Marty Friedman, Jason Becker and George Lynch are a few more great guitarists also Buddy Guy is no way Overrated he is absolutely incredible and live he is such a character!