I love this little tidbit from Craig Calcaterra at Hardball Talk:
A legal opinion has finally been issued on a very important matter.
Fricano asserts that the Cleveland Indians are “the arch rival” of the Chicago White Sox. While the two teams maintain a healthy rivalry, this court notes that it is generally accepted, at least among informed baseball followers, that the title of arch rival belongs to the reviled Minnesota Twins, to be shared, during inter-league play, with the Chicago Cubs.
So it is settled. The White Sox are the arch rivals.