Professor Bainbridge has produced a map of corporation laws in the United States showing which states have adopted the Model Business Corporation Act (MBCA). This is a nice resource for students to get their bearings reading cases in corporations or business associations. The cases themselves often don't mention whether their statute is based on the MBCA, but that information can be important to help students organize the cases in thinking about corporation laws. Of particular note for students, Delaware, California, Texas, and New York do not follow the MBCA, and they are some of the most significant states for incorporation. Kind of makes you wonder what the "majority rule" is, doesn't it?
Where do most companies incorporate? Like most things in law school, "that depends." For more information about where privately held companies tend to incorporate and where they tend to be physically located, see my analysis of private company incorporation decisions here. For similar data on publicly traded companies, see Bebchuk and Cohen's analysis of public company incorporation here.