Dear Paddutr,
There is no best answer to such questions. You simply cannot stereotype replies to interview questions. All situations are different, and the reply may be on a case-to-case basis. In most cases, the interviewer sees whether you are focused and confident and have a definite aim or objectives in your career.
For example, a person who is switching from a PSU to the private sector says that he is leaving the job because promotional opportunities and career growth are very slow. It is true because we all know that career opportunities and growth in PSUs are slow, and in most cases, seniority is the major factor in promoting.
On the other hand, if a person from the IT industry or marketing gives the same reply, the interviewer will take it with a pinch of salt or will make a lot of effort to believe it.
So, it is based on situations.
Regards,
SC