Depends on your tastes, really. For a more traditional Western (see my earlier post for some discussion about the distinctions), try just about anything by Louis L'Amour. Matt Braun also writes some fairly traditional Westerns. The Virginian is a must if you want to see the roots of the genre. For more historical stuff, Terry Johnston is good, but he tends to be a bit gory for some.
Richard Wheeler turns a good phrase, and for the more literary bent, Lonesome Dove is a must. Joseph West's Gunsmoke series isn't bad, but it doesn't stick to the TV series in some areas. It's also more traditional. Elmore Leonard wrote some good Westerns as well, though they're a bit more gritty than traditional.