What does success mean to you? 

Example:

"I have several awesome  achievements in my career. Probably the most awesome achievements were the delivery of the most recent version replace update to one of our core products for customer bills. This was a 1-year project and I was one of 9 team members. What made it more for me was that my role expanded from being one of the tech team individuals to taking the lead on building the mobile makings of the product. In order to deliver this aspect of the product, I pulled in 3 new project resources with new specific skills in each of the mobile technology floors we targeted and cross-trained all project resources to support each floor. In the last, our team delivered the product on time and have received numerous accolades, both external and internal. The mobile component was particularly  identified to be world class and distributed the product at its release. Our CEO spent time showing the mobile components to the industry and press and it has since met with superb  reviews. Would you like to see the Android version of the product?"

Focus on your most recent accomplishments, those in your current position or the job you had just prior to this one. But make sure they are relevant to the position for which you're interviewing.

This is a good question to highlight your achievements. Don't be smug or arrogant about the success of a project, especially if it was a team effort. You need to give credit to the team, too. You could say what you contributed to the total project and also give credit to others that helped in your success. Don't be overly modest, either.Here are some pointers:

1. Pick something that was clearly a success and highlight your skills.

2. Pick something recent. If you pick something twenty years ago, they are going to wonder what you have done lately.

3. Pick something important. You may have succeeded in many projects that are not that significant. 

4. Pick something easy to explain. If complicated, give the short version.

5. Be sure to explain what you learned from this success.