The way he says FULL. METAL. JACKET. is pretty haunting. Phenomenal scene. R. Lee Ermey and D'Onofrio were really so great in that film. I couldn't help but think (and I haven't watched the film in years) that it lost a lot after this sequence and that the latter parts of the film were not as good as the Paris Island stuff.
That said, I haven't seen it in a long time and there are many people who dispute that. I have to give it another watch because, at this point, my memory of it is mainly of individual scenes- this one, the opening with Ermey absolutely lacing into the soldiers in basic training, and of course the scene with Joker and the prostitute.
I think Apocalypse Now is the best Vietnam war film, but it also is more of an allegory about human nature than a straight up war movie. I'd imagine many people would question it being given that distinction because it is not explicitly about the war but more uses the war as a setting for its narrative.