Please tell me a little about yourself.

I know it's a very simple question, but it's very important to answer it. What is perfect answer to it?

there's no standard and so called perfect answer, it's just a way to summarize your background,

I agree there is no best way to answer that question. Think about it in this way: all your background is shown on your vita, why they still ask that question?

From my experience, I think you can't just summarize your experience or background. The reasons behind it are the things people really interested in.

For instance, if you have changed your major in school, why? If you have changed your jobs, why?

There might be thousands of reasons behind it (e.g. family problem, money etc.). But it is better to "figure" a reason related to the job you are applying or related to your career goal. Make it reasonable!!

Employers ask this question because they want to see how you made your decisions along the way. Focus on the transitions in the past and explain why you did it. For example, why you go to grad school, why start a job at company XYZ, why you want to apply for their company etc.