Surprised Shania hasn't been listed. Her husband Mutt Lange was one of the first that took pop/rock & merged it with country. From him you got the super formulatic tracks with layers of vocals, simple pop beats, and meterosexual Asian fiddle players. Garth was HUGE, but country. "Much too young to feel this damn old" is great, classic country. His stageshow wasn't "country" but his music sure was.
Earlier acts like the Mandrells also merged pop, but there was always a core of country that remained and desired. (See the explosion of George Strait's popularity). Since Shania it has been all downhill. There has been some resurgence as of late, but bro-country & the current crop of female singers have basically filled the soft rock/pop void that was replaced by stupid music like Drake, Post Malone, etc.
As someone who grew up liking classic country & classic rock.... music today sucks & it is hard to find much worth listening to.
With a Shania pic for rule 1: