There can be many answers to this question. I think that one would be accurate in pointing out that John Marshall, the nation's fourth Supreme Court Justice, was instrumental in constructing the court what it is today. The idea of Judicial Review, meaning that only the Supreme Court can determine the constitutionality of laws and actions, was an important construction. Justice Holmes' writings on the First Amendment, and the idea of the Constitution protecting the rights of all individuals are extremely important to the nature of basic freedoms. One could even argue that Roger Taney, specifically in the Dred Scott decision, was extremely important in demonstrating how the court could make dreadful mistakes that can have a profound impact on the nation's development.
Yet, I think that I find Earl Warren to be the one that stands out the most in my mind. In focusing on two decisions of the Warren Court, I think that Earl Warren's commitment to social equality and the rights of the individual proved to be two lasting legacies that showed the Supreme Court capable of great good in the American social and political fabric. Brown v. Board of Education was a decision that transformed American society. Our modern social order owes a debt to the decision and still has to strive to achieve what the decision set in motion. The fact that Warren was able to articulate a position that argued segregation being unconstitutional as well as the need for integration to happen "with all deliberate speed" (Supreme Court, 1955) are elements that both defined the promises and possibilities of American society.
In Warren's Miranda v. Arizona case, Warren upheld the idea that all individuals, even those accused of criminal activity, are afforded Constitutional protection. The notion here is that the Constitution cannot work for the betterment of all society if it is denied to specific members of that society. In my mind, I think that Justice Warren stands out because he saw the court as an instrument for social change when society was unable or unwilling to change itself. When legislators failed miserably to address what was in need of change in our society, Justice Warren saw the necessity to change it. In doing so, Warren stands out by demonstrating the power and function of the Supreme Court.