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.)
renoir is truly the master,he made it all look so easy&his films don't date,he &welles have probably inspired more great directors & films than just about anybody.also the great robert bresson deserves wider public acclaim if he made a bad film i haven't seen it & i have most of them!
Greatest director of all-time is a tough question. I noticed some people mentioned Japan, and I personally think Ozu is better than Mizoguchi and Kurosawa, but that's just me. Best American director to me would have to be Orson Welles, and best foreign (vague I know) director either Jean Renoir or Carl Dreyer.