The problem with saying "it wasn't a good fit" and leaving it at that is that you'll end up at another job with the same situation. What you want to do is say it wasn't a good fit, then explain why it wasn't a good fit, in as respectful a way as possible to your previous employer, but without papering over the problems.
Don't say things like:
- project madness
- no good analysis
- bad project planning and estimation
Do say things like (assuming all this is true):
- Due to disagreements between managers and employees, 15% - 20% of total employees have resigned.
- The latest project would require employees to work extreme hours (give a number, if possible) for X months, which I find unacceptable. I understand some crunch time is always inevitable, but I will not work planned crunch time for a long period of time.
- The timeline presented to me was unclear and, in my judgment, unrealistic, particularly given the recent resignation of so many employees.
In my experience, if you can give concrete, fact-based reasons why you left a company, without bad-mouthing that company, you can paint a picture to your interviewer that it was an unhealthy place to work without alienating that interviewer. And, if that interviewer has a problem with you not wanting to, for instance, work crunch time for months, then you don't want to work at that company.
What people often forget while interviewing, is that you're not trying to get this job, you're trying to get the right job. Therefore, it's important to be clear about what you won't do in an interview. Just like the employers want to weed out applicants, you want to weed out all the companies that are a bad fit for you.