A recent study of citations to state supreme court opinions reveals that the California Supreme Court is most often followed by sister state supreme court.  The New York Times article describing the study includes the following:

In the 65 years ending in 2005, more than 24,000 state high court cases have been followed at least once. California leads with 1,260 decisions. Washington is next, with 942, and Colorado is third, with 848. New York comes in 10th and is only about half as influential as California, with 627 followed cases.

The study, published in the UC Davis Law Review is available here.