Depends on genres and their level of profile. There are many top players in the jazz, bluegrass, country and folk areas but these are 'specialist' non-mass market tastes and so tend not to be known in the outside world.

In the rock and blues world I'd say the most criminally underrated/unexposed players are

Danny Gatton - could play almost any style brilliantly
Hellecasters - Will Ray, John Jourgensen, Jerry Donahue. These 3 could play the arse off anyone out there, no problem.
Tom Morello - the sounds he can get out of his guitar
Skunk Baxter and Denny Dias - their work with Steely Dan. technique, taste, tunes..all there.
Elliot Randall - for the solo on 'Reelin in the years'
Dicky Betts - it was him on 'Jessica' not Duane.
Roy Buchanan - same as with Danny Gatton
Mike Keneally - every bit as good as Satch, Vai etc

Overrated - depends who's doing the rating. you've gotta be objective about this and not write off players just cos you dont like their material. I dont go a bunch on Dream Theater, Malmsteen, Vai, Pantera, Zep & Yes but I recognise theyre decent players.