All Quiet in the Western Front is a good war film (1979, about WWI)
The first one is way better, but a bit old (1930).
I also recommend everything that has "Kubrick" or "Tarantino" written in the credits, no matter where. Everything they approach turns gold.
I didn't look at all the previous posts, but here are some movies pretty watchable:
City of God (the 2002 movie) that recounts the evolution of gangs in the favelas from the 60' to the 80'. One of the best endings evar.
Children of Men; women can't make babies, immigrates are illegal, world collides. All this made by some talented mexican filmmaker through awesome sequence shots.
Oh, and I just noticed that the better a movie is, the more awful its trailer gets.