True Grit!
In this group, it's between Big Jake and Rio Lobo, but since you've already watched El Dorado, Big Jake is the obvious choice. Rio Lobo, Rio Bravo and El Dorado are all basically the same movie (NTTAWWT if it's John Wayne).
I always liked Rio Bravo over El Dorado because to me Rickey Nelson trumped a James Caan that couldn't shoot! That an Angie Dickinson trumped Charlene Holt as the chick!!!
Rio Lobo has a special father/son meaning to me. In '70 or '71, as ten or eleven year old, I took my father to see the movie at the local theater for Father's Day.
Love the running joke in Big Jake when just about everybody McCandles meets thinks McCandles was dead.
http://www.imdb.com/title/tt0066831/quotes