In case you didn't already know, outside of the Bronx, the New York Yankees are easily the most hated baseball team in the United States.
The results are pretty predictable.
The Yankees are the most hated team in the majority of states. The Boston Red Sox are the most hated team in four states, two of which are filled with Yankees fans. Missouri also hates the Red Sox, but that's only because Boston has beaten the St. Louis Cardinals in the World Series twice in the last 10 years.
More data, such as the most hated team by province in Canada, can be seen at Imgur.