Here's a spur-of-the-moment list of five.  (The order here is very roughly preferential.)

Dreyer
Lang 
Mizoguchi
Ozu
Renoir  (I haven't seen anywhere near as much Renoir as I ought to have seen, but I have an excuse for this deplorable ignorance on my part:  most of his work is (I think surprisingly) hard to find here in the U.S.  I don't doubt that I'd think him even greater than I do if I were more familiar with his work from the 30s.  Sadly, it may be a while before I find out just what sort of effect this would have on my opinion of his oeuvre.)
