Can't go wrong with the outlaw guys many have mentioned.... Waylon, Willie, DAC. Willie Ray Hubbard, Robert Earl Keen also both good. Shooter Jennings
More roots, americana artists.. Old Crowe as mentioned are great, awesome live. Ryan Bingham, The Avett Brothers, Felice Brothers, Deer Tick, Justin Townes Earle (and of course his pops Steve), Slaid Cleaves, Drive By Truckers, old SteelDrivers.
Some newer more blue grassy bands.. Railroad Earth, Yonder Mountain String Band, Greensky Bluegrass, Trampled by Turtles, Devil Makes Three, .357 String Band, Chatam County Line, Grandpa's Cough Medicine
Oh and anything Keller Williams does.