I'm a big, big, big fan of Lee Hazlewood if you know him? It's not really country, but more country-pop. His earlier work (early 60's) is pretty country, but he changed into more of a pop sound during the 60's. The reason I like him is because of his slow sad songs, as they are almost always superb! I think the only songs I like of his uptempo country songs are "Poet, fool or bum" & "Big red balloon". He also has some kind of midtempo kind of country stuff on his "Lee Hazlewoodism - It's cause & cure" record. They're also good, except for "Suzi Jane is back in town" and "The girls in Paris".
I haven't heard that much Johnny Cash, but I think "Ring of fire" is alright. Maybe because it's kind of fun.
I have a record by Merle Haggard, but the only song I like on that is "Song from "Sleep walk"". If I'm not mistaken it's from a film.