I don't listen to a whole bunch of country, but of the times I've let myself fall under the spell of the twang, these stood out the most:
Dwight Yoakam (although he's more Honky Tonk than Country)
Willy Nelson (his vocal timbre is unmatched)
Roy Clark (virtuosity on many instruments)
HM's: Johnny Cash, and also Kris Kristofferson for his songwriting.
https://www.youtube.com/watch?v=IRU9i9egr7A