I guess I would put him the blues category for this thread, but Townes van Zandt never made a bad album. I highly recommend the movie Heartworn Highways for some good outlaw country in the '70s.
John Prine's first album is almost perfect as well. Another person who I think it underrated is Steve Goodman.
Ten Years After put some great stuff, as did the Paul Butterfield Blues Band.
Big Star has been getting some love lately.