Dwight Yoakam.
Guitars and Cadillacs is brilliant. Some of his newer stuff, like Blame the Vain is quite well done as well.
Also, Old 97's first couple of albums are rollicking, fun alt-country. Neko Case, k.d. lang, and Gillian Welch are all more or less country on different albums (and country in different styles). 16 Horsepower and Woven Hand, the solo project that arose from it, are great countryish and very dark Americana.
Finally, I'd like to second the Johnny Cash, Merle Haggard, Gram Parsons, Uncle Tupelo, Son Volt, Hank Williams, and early Okkervil River and Wilco suggestions.