With a metric that few people would even dream up that they call the Fan Difficulty Index, the Kansas City Business Journal has developed a formula that rates the Kansas City Royals as the third most difficult club for a fan to cheer for based on certain standards of winning (or not winning as KC's case may be).
The Yankees currently sit in first place in terms of ease for a fan to root for them, given their milestones of historical winning, timeline since last championship, etc. The Washington Nationals are the most difficult, which doesn't seem like a stretch for anyone who's been a fan since they moved or even during the Montreal Expos days.
You can find a full explanation of the metric they use here, and you can find the complete list of teams here.