What should the best answer "why you are leaving your current job"?

Although anything you say can and will be questioned further, as per my experience the best answer will be as follows:

1. My company is shifting me to a remote location which I am not confortable to go to, or

2. My company is changing my role but I am expert in my field and I want to make a carreer in this, that is why I am interested in your organisation, or

3. The project on which I was working is finished and I don't want to be in uncertaininty about my future work so I am looking for a change.

Stay positive regardless of the circumstances. Never refer to a major problem with

management and never speak ill of supervisors, co-workers or the organization. If you do,

you will be the one looking bad. Keep smiling and talk about leaving for a positive reason

such as an opportunity, a chance to do something special or other forward-looking reasons.

The best answer is really answer

Like improve my self

There may be one or multiple of following factors:

• Changed preference for location
• Change in location of posting
• Better opportunity elsewhere
• Disharmony with technology in the organization
• Change of Role
• Change in Management Heirarchy and Supervisory Managers 
• Decrease/Limits on Perks etc