Marbury v. Madison is probably the most important turning point case in the Supreme Court's history as it, arguably, gave itself the power to review law for constitutionality with its ruling.
By establishing judicial review, the Supreme Court gave itself a mission that would define its history for the remainder of its existence.
Dred Scott is a poorly decided case that caused a war but was not a major turning point for the Court. In fact, it took many years for the Court to recognize the error of its ways.
