For years, rooting against the New York Yankees in October has forged a common bond among Yankee haters nationwide. But this year, the Yankees failed to make the postseason for just the second time since 1993. As a result, many Americans are grappling with a vexing question as the playoffs get going this week: Which team should we hate now?

To help solve this existential crisis, The Wall Street Journal created the Major League Baseball Hateability Index, which ranks the nine teams that were still in the postseason as of Wednesday...