One way to judge might be to consider which casebooks played a mjor role in getting their particular subject widely adopted as a class in American law schools. Among the top contenders might be: Ernst Freund, Cases on Administrative Law (1911); and Richard W. Jennings & Harold Marsh, Securities Regulation: Cases and Materials (1963).
Ranking even higher, I would suggest, would be a casebook that not only get the subject into the law schools, but plays an important role in creating new lawyers who will, during their careers, significantly change the existing law on the subject. On the real-world influence scale, can anything top Ruth Bader Ginsburg, Herma Hill Kay & Kenneth M. Davidson, Text, Cases, and Materials on Sex-Based Discrimination (1974)?
Commenters, what do you think should make the honor roll of most influential casebooks of all time?