This is a question that I actually think is challenging!

If you're talking about the quality of output, throughout the history of film...I think I would have to say Sweden.  Not because they make more "masterpieces", but because they seem to only export their "best".  I wish more countries would show the restraint the Swede's do when foisting their films on the rest of the world (take that FRANCE...You too ITALY!)

Sjostrom, Molander, Stiller, Bergman (Ing & Dan), Troell, Christensen, Sjoberg, Widerberg, (early) Hallstrom and Moodysson, etc.!  They may not make a lot of films, but they make a lot of good film directors.