What is the best tool for creating and maintaining content calendars?
CoSchedule impressed me for the fifteen months I used it. While the interface seems average at best, CoSchedule's functionality seemed way above average, especially for its cost. From the calendar, you can publish to WordPress, post to Facebook, Twitter, and LinkedIn, and map out all the content your team needs to publish (the ability to create custom content types was helpful); you can also share the calendar with your team members without giving them "seats" on your subscription.
The "best" tool will depend on what you're trying to achieve and your budget (spreadsheets can be effective and free).
I've used Asana and Wrike as more robust tools for project management and editorial calendars. I found both to be helpful in managing not only the editorial calendar, but also various freelance writers.