I think the schools that produce the most case studies are; HBS by far is #1 and some of them are legends that almost all schools use. Then I forget the order but Ivey, Darden, and Kellogg. I know Kellogg is doing a big push to create more and I was surprised to see numerous Ivey case studies along the way in some classes (they are very good too). Darden is an all case study school so it makes sense the produce a lot too.
I love kellogg cases since they are free and free is always good.