well depends on what genre your looking at. another thing is, there are soo many songs that go unheard by the general public. I've been lucky enough to have been introduced to this "underground" alternative country-rock. I encourage anyone with an open mind and a true appreciation of music I think you should check out Ryan Adams, Jason Isbell/Drive-by Truckers, Will Hoge, Uncle Tupelo, Son Volt, Cross Canadian Ragweed etc.

now back on subject...
a few of the greatest songs in my opinion
"Pancho and Lefty" written by Townes Van Zandt, the most well known version done by two greats Willie Nelson and Merle Haggard
"He Stopped Loving Her Today" by George Jones
"The Cowboy Rides Away" and "Amarillo By Morning" by George Strait
"Sunday Morning Coming Down" written by the great Kris Kristofferson, done by Johnny Cash
"Loving Her Was Easier (Than Anything I'll Ever Do Again)" another written by Kristofferson and done by Waylon Jennings

there are soo many more songs that are great and pure, its too many to name