Driver started a list for Top Letterboxd Westerns. See instructions at letterboxd.com/drivefiction19/list/the-letterboxd-communitys-50-favourite-westerns/. I know there are several essentials of the genre that I still haven't seen. Still, I admit to being highly idiosyncratic in my selection. For example, I've seen many Clint Eastwood westerns, but none make my top 10.
Seems a shame not to have Roy Rogers or Gene Autry represented, but no one picture of theirs stands out. I haven't seen enough of William S. Hart, Billy Boyd, Tom Mix, Tim McCoy, etc to be a judge on those.