One of the most difficult

One of the most difficult and most frequently asked questions is the following:
QUESTION: Why did you leave your last job? The real fear behind this question is that you may have been fired or that you are just changing jobs on a whim, only for the money, or that you are a “job hopper.” Let’s look at three alternatives for fielding that one:
ANSWER A: I became aware that there were some excellent new opportunities in the field of biotechnology. I really wanted to expand my professional growth by finding out more about them. This company, judging by your Web site, seems like it would have some interesting opportunities.
ANSWER B: I felt pretty bored at my last job, so I just wanted something more stimulating.

ANSWER C: My last boss and I really didn’t see eye to eye. He wouldn’t give me a raise no matter how many times I tried to get one.

I bet you already chose A. You’re right! No matter what question you are asked about your last company, your last supervisor, or your former coworkers, the intent is usually to “dig up” something negative. Both B and C imply some sort of negative response to your last job.

Sharing ANY kind of negatives about a past employer is to be avoided at all costs, even if you feel that you were treated unfairly.

Taken from : Fearless Interviewing - How To Win The Job