I think the case can be made that it's Oliver Wendell Holmes, Jr. Among the important original legal doctrines and concepts that he took the lead in articulating: Importance of freedom of speech and the thought experiment shouting fire in a crowded theater. read more
Besides placing a close second to Oliver Wendell Holmes in Brian Leiter's Most Important American Legal Thinkers, he holds the honor of being the most cited legal scholar of the 20th Century. Shapiro, Fred R. (2000). read more