Let's say they had to make at least three films/projects together to count.
Considering the sheer number of great films, I would say it has to be Kurosawa and Mifune, but maybe I'm forgetting someone. Wong Kar-wai also has several great collaborators, so many that I didn't know which one to choose for the poll. There's also Leone and Eastwood, of course, and Tarantino and Jackson, but there's not enough room for them either. Some others that come to mind are Hitchcock with either Grace Kelly or Cary Grant, and Elia Kazan with Marlon Brando. Seven choices is not enough for the kind of polls.
Considering the sheer number of great films, I would say it has to be Kurosawa and Mifune, but maybe I'm forgetting someone. Wong Kar-wai also has several great collaborators, so many that I didn't know which one to choose for the poll. There's also Leone and Eastwood, of course, and Tarantino and Jackson, but there's not enough room for them either. Some others that come to mind are Hitchcock with either Grace Kelly or Cary Grant, and Elia Kazan with Marlon Brando. Seven choices is not enough for the kind of polls.