This question is never fair because most hiring managers know that people leave a person (usually a boss) first, rather than a job role. This reason is generally followed by lack of opportunity/development within a team, a mismatch of company culture and possibly followed by money, benefits and work-life balance. The order may change but these are general reasons why people leave jobs.
At the end of the day, beyond your capability to do the job most hiring managers are focused on how you can make a difference in the job role they are looking to fill and how you will integrate with the team and fit with their company culture. Therefore, you should reframe your answer around these key areas. You do not need to go into any detail about your old boss but mention the type of environment and job role and career development you are looking for now. If you have done your job search preparation correctly the organisation you are interviewing with should align with your aspirations and future development ambitions which should be the reason you are interested in working with them and not some other organisation so it’s important to clarify this point when you answer the question.
If you are asked directly about your relationship with your boss say that you learned about yourself by working for this person, for instance, if your boss was picky and critical, you should say that you learned to be more detail orientated.
If your old boss was a poor planner, you could say you learned to work flexibly and always made contingency plans to ensure that any lack of planning didn’t hold up delivery of project outcomes. Finish off your examples with “…but now I am looking for a team, culture or boss that has more of (X, Y and Z).”
If you are in any doubt about answering this question. Remember this; no organisation wants to take on someone who is a whiner and who is critical of others so always reframe your answer around learning outcomes and your contribution to the job and don’t be tempted to bad mouth your ex-boss, even if they were awful to work for. Keep your answers positive and forward-looking.
This post has been adapted from an earlier post that was answered on http://www.quora.com