The easy part of your response is to assert confidently that you do consider yourself a success. Make sure you look the recruiter in the eye and sell the statement with a confident tone, but without bragging. The more challenging task, however, is to back up your assertion. It's important to provide the interviewer with evidence of how you achieved success in the workplace.
Provide one or two examples of times when you have set and met a professional goal. Briefly explain how you achieved each success – perhaps you overcame an obstacle, effectively managed a team, or budgeted your time effectively. The goal is to demonstrate your determination and willingness to take on challenges and achieve results.
You might also mention successes you hope to achieve in the future or are currently working to achieve. For example, if you mention your successful sales record, you may also want to explain how you hope to improve upon that success in the future. It will demonstrate that you are hungry for new challenges in the new position.