Written in jail by a 19 year old Lefty Frizzell to his wife Alice, I Love You a Thousand Ways is widely regarded as the very first modern country song, but not the 1950 version, the 1959 re-release.
It’s hard to say what the very last true twanger standard was, but Lefty’s 1973 I Never Go Around Mirrors is often mentioned as it.
Personally, I give the nod for last true twanger blockbuster hit to Vern Gosdin That Just About Does It in 1989. It’s 100% Lefty imitation though.