The most important goal for a question like this in a Stanford MBA program interview is to convince your interviewer that you know what you want out of the program and that you understand how the unique offerings at Stanford align with your goals. For example, you might describe your desire to work with a specific faculty member. You might mention classes that aren't offered by the other programs you have researched. Go beyond a simple 'Stanford is the best' response and offer a specific reason you choose to apply at Stanford. Answers that meet this standard are more likely to resonate with your interviewer.
You can think of your answer to this question as your elevator pitch for program acceptance. You are likely to be asked this question more than once throughout the selection process and you want to offer a clear and compelling response.