Which are the best state high courts? It depends on what you mean by "best." Suppose you think the best courts are those that write the most influential opinions -- are cited the most by out-of-jurisdiction courts. Then the best are California, Delaware, and Montana. The worst are Missouri, the Texas criminal high court, and the Oklahoma criminal high court. (Texas and Oklahoma have separate civil and criminal high courts.)
Suppose you think the best courts are those that publish the most opinions. Then the best are Georgia, Mississippi, and Arkansas. The worst are North Carolina, Delaware, and New Mexico.
Maybe the best courts are those that are most independent (which we define to mean that partisan differences among judges have little effect on outcomes). Then the best are Rhode Island, New York, and Oregon. The worst are Connecticut, Indiana, and Michigan.
And overall? If you (arbitrarily?) give equal weighting to the three measures, the best are California, Arkansas (!?), and North Dakota. The worst are Missouri, the Oklahoma criminal court, and Michigan.
Such is what I found, with two colleagues, when we crunched the numbers taken from 1998-2000. One might compare our rankings with this one and this one. What do practitioners think? If you have practiced in front of any of these courts (or have a sense of their reputation), please let me know in the comment section what you think of these rankings. (Remember: high courts only, not the entire judicial system for a state.) Judges seem not particularly happy about them (except for these ones).