I’d add Harvest Moon by Neil Young, just an absolutely beautiful song.
I’d also add some Hank Snow, Red Foley, Ernest Tubb and even though it’s bluegrass I’d definitely put in Cripple Creek, Foggy Mountain Breakdown and Orange Blossom Special.
I do agree that I’m So Lonesome I Could Cry is the greatest country song of all time, though!
Elvis sang I’m So Lonesome I Could Cry during some shows. He said it was the saddest song he ever heard.
I'd add a couple more that hardly ever get much radio airplay:
"Long May You Run" (it finally got some recognition when they played it during the final credits of the movie "Miracle" about the 1980 U.S. Olympic hockey team)
"Harvest"
"Winterlong" (maybe his most underrated song)