They Were Expendable is my favorite.
Here's some trivia for you: The Green Beret was the only Vietnam War movie made during the Vietnam War. If I'm not mistaken, John Wayne paid for the production of that movie out of his own pocket - need to double check on that.
But the plot is very timely...a cynical anti-war reporter goes with Wayne and his Green Berets into Vietnam. Afterwards, he changes his views about the war.
"What are you going to say in that paper of yours?"
Ah, if only art imitated life.
Sands of Iwo Jima is my favorite John Wayne movie.