What is the best answer to Why did you quit your job question?

What is the best answer to Why did you quit your job question?
Probably the best answer is to tell the truth, but as an employer you want to make sure that the person isn't just flaky and quit for a silly or irresponsible reason. If you quit so that you could go back to school, or if you quit because you needed to move to a different state, those would be nicer to hear than something like "I hated my boss" or "I couldn't deal with the politics anymore." Although those are good personal reasons to make a change, there are personality conflicts and politics anywhere, and people will worry that you didn't put in the time or effort to overcome these things on your own. So, if that was the reason, it might be wise to talk about all the things that you tried before you gave up.