After the climactic showdown, Jeb is sitting on a porch with his wife, played by Patricia Neal. As they watch the sun setting, Jeb sings a couplet of a folk song:
*"The sun is going down on the plains,
And my cattle are in,
You'll never know what this freedom means
Until you've been in it, you've been in it..."*
He doesn't finish the song, instead, the scene fades to black.
So, technically, the last *spoken* line in the movie is not from John Wayne, but is the final line of dialogue spoken by Patricia Neal, who says "Well, that's all I need." just before the scene where Jeb sings.
However, the last line *heard* in the movie is the sung line, which makes it arguably the most memorable "last line" of the film.