All of those other movies are 10 out of 10's themselves but they either don't capture Vietnam in the same way Platoon does or Vietnam is just the setting for a larger story. The second part meaning- you could have put the story into any modern American war and it would have worked the same. Platoon was uniquely Vietnam.
Full Metal Jacket was a great movie (and I'm a HUGE Kubrick fan) but it wasn't really a movie about Vietnam. The first and most memorable half of the movie is set stateside in a boot camp and makes no mention of Vietnam. The first half could be set in a WW2, WW1, Korea etc. movie without a problem. The second half of the FMJ, the Vietnam part felt rushed and incomplete. The only lasting memory most people have of the second part is of the "Me love you long time" opening. I think it's safe to say FMJ isn't so much about Vietnam but is more a criticism of war and the military than anything.
Of the big four Nam movies, Deer Hunter is probably the least about Vietnam than anyone of them. Of all the Vietnam movies Deer Hunter is probably the best 'movie' in and of itself but less than half of the movie is actually set in Vietnam. Most of it deals with the relationships of the the Vets before and after the war. And while the two most intense scenes from any of the aforementioned movies came from this one, the fact that only a quarter of the movie is in Vietnam doesn't really qualify it as a Vietnam movie. If someone asked you for a movie about Vietnam, Deer Hunter would be the last movie of the four to give them.
Apocalypse now is the only movie which comes close to rivaling Platoon in terms of what a 'Vietnam movie' would be. But it loses it on two points. The first would be the actual plot basis of the movie. The movie isn't so much about the war as much as the war is a back drop for the assassination of Marlon Brando-Colonel Walter E. Kurtz. Like Full metal Jacket, the movie could very well have been set in Korea or WW1. The second point where the movies loses it is the ridiculousness of some of the scenes. I'm referring to scenes like Dennis Hopper's rant or the surfing Colonel. I can appreciate those parts of the movie for what they are- good movie. But when it's not authentically Vietnam. It 's not accurate as to what the war was like. I think the movie is attempting to be both a criticism of war and a criticism of Vietnam and manages to not do either really well.
Platoon manages to be authentically Vietnam while still providing an excellent storyline with the two Sargents and their effects on a young Sheen. The movie manages to use the main plot of the two Sargents and their dealings with Sheen to actually showcase what Vietnam was in a way Apocalypse Now couldn't. For instance, Sargent Barnes murder of the woman in the village actually builds on tension between Barnes and Sargent Elias and at the same times gives a glimpse of what the reality of Vietnam was like. While the movie occasionally gets into the relationships between the soldiers they are all still kept within the context of the war. Neither the side tracking philosophizing and introspection of Apocalypse now nor the relationship focus of Deer Hunter is present in Platoon. The movie stays grounded in it's location and theme. The theme of Platoon is Vietnam not anything else. That's why if ever someone where to ask you for a movie about Vietnam, Platoon would be the perfect movie.