Platoon was a genre changer, and set the stage for many war movies after it.
Apocalypse now was a good movie. But I do not consider it a "Vietnam movie". It was a fantasy movie set during the Vietnam war, but obviously had nothing to do with reality.
Full Metal Jacket - like someone else here said, was almost two different films. The training first half was awesome, but not really anything to do with Vietnam. The second half was a pretty unrealistic meandering with a few war scenes thrown in.
Born on the 4th of July was more about the after effect of a seriously injured war vet. Suffering PSD, trying to fit back into civilian life as well as the conditions of the VA Hospitals.
In my book... probably Platoon. But even it was more about the fight between two sergeants than the war.