Seconding Husker Du! Good choice.
#1 Under-Appreciated Artist goes to Jeff Magnum of Neutral Milk Hotel.
Post-Rock - Talk Talk (late 80s early 90s)
Folk Rock - Pearls Before Swine (late 60s?)
Indie Rock - Circulatory System (mid 00s)
90s Rock - Brainiac (mid to late 90s)
Reggae - Linton Kweli Johnson (70s, 80s)
Soul - Donny Hathaway (late 70s,early 80s)
Rap - Del (present)
Bluegrass - Trampled By Turtles (present)