Off the top of my head I could name a bunch of guys better than him.
Hank Williams SR & JR, George Jones, Johnny Cash, Willie Nelson, Merle Haggard, Conway Twitty, Dwight Yoakam, Buck Owens, etc..
If you want a good bluegrass band, then check out Iron Horse. They have great original music plus they've done covers of Metallica, Ozzy, and Zepplin. I'm actually having them play at my wedding. They're good.
Kris Kristofferson has probably impacted country music more than anyone mentioned so far in this thread. Country is defined by it's songwriters, and he's one of the best.
I switch back and forth between Hank and Cash being number one. They're both great. Regardless of how they're placed, Cash and Hank are number 1 and 2.