For classic country, it's a toss up between Waylon Jennings and Merle Haggard I also love Tom T. Hall. His songs are fun sing-a-longs.
For modern country, I've recently developed an infatuation with Keith Urban
Oh and this just reminded me, the very first album (it was a cassette tape actually) that I remember buying with my own money was the "Poncho and Lefty" album that Merle and Willie Nelson did. I was only about 7 years old but I so loved that song. I think I still have that tape. lol I heard the song the other day on the radio and had to stop what I was doing and just listen.