Probably Richie Furay's "Kind Woman" on the third Buffalo Springfield album 1968 (I don't think of "Sweetheart of the Rodeo" 1968 as country ROCK; a few years later, Gene Clark's "White Light" 1971 (I was late coming to Clark though not too late to the Byrds).

Most people wouldn't consider it "country rock" but John Cale's "Please" from 1970 has a luscious steel guitar and a Floyd Cramer / Hargus Robbins style piano (and Cale's characteristically surreal (not country) lyrics. https://youtu.be/uto4s3oLmnU