When I saw the OP, I approached it in terms of ‘American’ Classic - either films of a genre perfected by American cinema, or films that broke ground in terms of the American psyche.
In terms of the first category, that would have to be The Usual Suspects. The crime genre is one that American film-makers do particularly well (GoodFellas, Casino, The Godfather(s), LA Confidential, etc, etc). I’ve seen it countless times, and it still keeps me enthralled. Brilliant characterizations; crisp, clever narration & dialogue; a collection of some of the most versatile actors around, and a construction leading to one of the biggest cinematic “aaaahhhhhs” in memory.
As far as the second category goes, and as much as I am not really a fan of his films in general, it has to Platoon. Despite the fact that Oliver Stone has disappeared into his own personal delusions, perhaps never to return, his bigscreen debut made it OK in the US to discuss and confront the Vietnam legacy. Witness the many pretenders in it’s wake: Hamburger Hill, Full Metal Jacket, Tour of Duty (yeah, yeah, I know - TV show, but it just illustrates the point), Bat21, and all those terrible Rambo movies. While it certainly didn’t inspire quality follow-ups, it did make the subject presentable in the popular media and the public consciousness. Apocalypse Now did predate it, but the commentary on the madness of war kinda diverted any discussion on the common experience of Vietnam. Still a good movie though.
Plus, I gotta disqualify Full Metal Jacket on the traditional nitpicky grounds of book-to-film transfers. Based as it is on Gustav Hasford’s brilliant novel “The Short Timers”, Kubrick over-reached himself in his attempt to staple his own message over the much bleaker one conveyed in Hasford’s book.
Honorable mentions go to Clerks, the hands-down best piece of independent filmmaking in at least the last 20 years, as well as Schindler’s List, which could easily replace about 70% of the history taught in schools.
z