The Virginian and Ox-Bow Incident are good choices, JD.
In addition, I would definitely recommend the work of Thomas Eidson. An excellent writer and all his books share a theme of strong women in the west. Something that I suspect would be useful to include in a college course of this type when so much of the literature will be "male-centric". Charles Portis' True Grit could also fall into this category, as could Glendon Swarthout's The Homesman. All good books with plenty to discuss packed into reasonably short texts. (a bonus for any student and teacher alike)
In terms of 'popular' western writers you can't go past Elmore Leonard for my money. Not only are his westerns well written they also, I believe, are cinematic in style and their adaptation to the screen (often by Leonard himself) should give some interesting opportunities to compare and contrast the west of literature and film. Also, Leonard is a good example and measuring stick of the shift in American tastes in regards to the western genre. He started writing almost exclusively westerns in the 1950s because he perceived a constant demand for such stories but then switched to crime fiction during the 1960s when this market was steadily drying up.
Lastly, perhaps something that interweaves fiction with factual characters and events might be of interest. Thomas Berger's Little Big Man springs to mind as an obvious example. Depiction of characters such as Custer and Hickock for example as well as the 60s revisionist view of the west.
Just some ideas anyway from the top of my head.