For some classic stuff:
Ralph Stanely
John Hartford
Earl Taylor
Flatt and Scruggs
Nitty Gritty Dirt band
Clarence "Tater" Tate
Bill Monroe
Don Reno
Log Cabin Boys
Old and in the Way
Newer stuff I suggest David Grisman quintet(even though there more acoustic jazz than bluegrass), Steve Earle and the Bluegrass Dukes and the Yonder Mountain String Band.
If you dig Del McCoury, I would suggest getting the "the Mountain". It's the album where they collabrate with Steve Earle and it is fucking tits.
