Johnny Cash, Merle Haggard, Dolly Parton all wrote their own songs, Willie Nelson too. That places them higher on the list for me. That is being a more of the full package.
If the author has to exclude whole genres, Bluegrass which Wildwood Flower is somewhat related on or San Antonio Rose by Bob Wills for Texas/Western Swing, that's okay. "Foggy Mountain Breakdown" by Scruggs and Flatts is legendary, I don't know if it is outright bluegrass but it's certainly related.
Yes, I knew Jim Reeves was more of a rockabilly singer in his early career.
I still say, the author basically named the top artists and one of their top songs.
At the least, with all of the sub-genres of country music, some even being rockabilly or more like rock music, I'd say something like the 12 greatest hits from 1980-2019 or something,
A lot of known pickers in the video below of "Foggy Mountain Breakdown":
Sorry...The long haired hippy looking guy seems to be the only one who doesn't really play that main riff, others in the video are Scrugg's son, the Osborne Brothers and I'm sure others.
Elvis and Johnny Cash and many others made Sun Records legendary, you really shouldn't pick a non-Sun records song for Johnny Cash imho and he wrote a ton of great songs when he was with them, then, in the '60s and there's nothing wrong with it, he started catering to a somewhat more broader audience.