To me, Hag is the greatest "country" singer ever and the only one that I ever actually refer to as country most of the time. All the rest (even the ones I love) are to some degree a hybrid of other styles. I won't even get started on "pop country" as I don't understand how including the word country in the description of 99.99% of what is recorded today is still occurring.
I think the last couple of pretty traditional country (not alt country or Texas or any other hybrid - just traditional country) artists that I would generally classify as such are George Strait, Randy Travis and [old] Alan Jackson.
George Jones is almost definitely the greatest country vocalist ever IMO. His versatility was simply amazing. Going from the old style upbeat stuff like "The Race is On" or "White Lightning" to the ballads is no small task, and he did it better than anyone. He also is one of the greatest (if not the greatest) harmony vocalists that the genre has ever seen. I have been playing and singing for over 30 years, and George was one of those folks that you just revered for his talent.
RIP George. You've earned the break.