There is no definitive answer to this question as it depends on the legal system in question and what area of law you are interested in. However, there are some well-known cases that have had a significant impact on the development of the law. In the United States, for example, Brown v. Board of Education is considered to be one of the most important cases as it resulted in the desegregation of schools. Another significant case is Roe v. Wade, which legalized abortion nationwide. These are just two examples – there are many other important cases out there that have shaped different areas of the law.