That's the question Professor Gerald F. Uelmen asks in "Who is the Lawyer of the Century" (Loyola of Los Angeles Law Review, Vol. 33, 2000) (PDF). The article, which includes a number of data points, concludes that Clarence Darrow should get the honor. The data points include a survey of lawyers and law students, whose first and second choices were as follows:
Death Penalty Lawyers: Clarence Darrow, Thurgood Marshall
Arizona Criminal Defense Lawyers: Clarence Darrow, Gerry Spence
Law Students: Clarence Darrow, Johnnie Cochran
Some of the contenders for the honor were asked what they thought about the century's best: Johnnie Cochran chose Thurgood Marshall; Gerry Spence chose Clarence Darrow; and Alan Dershowitz and F. Lee Bailey both chose Edward Bennett Williams.