Glen Campbell
Loretta Lynn
Patsy Cline
Justin Townes Earle - [Alternative Country]
Steve Earle - [Alternative Country]
Neko Case
k.d lang - [She has dabbled in country successfully]
Gretchen Wilson
I recommend Revolution by Miranda Lambert. Its an excellent album, very accessible. Also Jason Aldean's My Kinda Party. A couple of my favourite albums of the last couple of years Easton Corbins debut album and the last Billy Currington album.
Another overlooked artist who is very talented is Matraca Berg often a session songwriter for others but she does have a few albums of her own, her new album The Dreaming Fields is really good. My hubby is in the music industry and he was lucky enough to work with her some years back.