Sure, best directors is not good, but it's a better way of classifying films than by nationality. The fact that anyone will have their own personal opinion is beside the point: this is the case in any situation.

In short, my point is that it may be useless classifying the films of a certain country together. For instance, Mon Pere, Ce Heros is far removed from La Haine, which in turn has not much in common with Jules et Jim. I only liked one of these three films, yet (correct me if I'm wrong) they are all from the same country.