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 confidence and have a definite aim or objectives in your career.
For example a person who is switching from a PSU to Private Sector says that he is leaving the job because promotional opportunities and career growth is very slow. It is true because we all know that career opportunities and growth in PSUs are slow and most cases Seniority is the major factor in promoting.
On the other hand if a person from IT industry or Marketing gives the same reply, the Interviewer will take it with a pinch of salt or will take a lot of effort to make him believe it.
So, it is based on situations.
Regards,
SC