There should be three parts to your answer:
1) You can achieve great results
2) You're a good fit for the team and the corporate culture
3) You're a better hire than all the other candidates.

If your results at work in the past have been excellent, it's likely they'll be good in the future as well. In other words, someone who was good yesterday will probably be good tomorrow. So, think about your past work and present your best results (e.g. you took action X and managed to increase revenue by 10%).

Go to the company's website and learn about their values. Don't forget to check Glassdoor for some "real" opinions. Reflect on your own values and beliefs and show that these are largely similar to the company's values.

At the end of the day, any hire is a "business case" for the company. That is why you should demonstrate very clearly that the value you add to the company is much higher than your salary.