"I previously worked for a bank that needed to establish consistent and reliable data to be used for decision-making purposes. As the leader of this project, I determined that we needed to decommission multiple legacy systems, hundreds of spreadsheets, and several other tools and platforms in order to standardize the data. Once the scope of the project had been laid out and I had run a RAID analysis, I put all the details of the project into an online project management program, assembled teams, set expectations, and established timelines. I communicated clearly to the various parties that participated in the project, from upper management to the IT team and other employees. Ultimately, the project was completed within budget and on time, and it allowed the bank to standardize its data, improve its transparency, and reduce costs."

Explain the details of the project, your roles and actions, and the final result. Provide enough supporting detail to convince the interviewer that you can handle complex projects with ease. If the reason you selected the project as your most successful isn't obvious, end by explaining why you chose the project you selected to discuss. (i.e. the project was delivered over budget, but the new feature your team worked hard to include led to an extra 75% ROI)

Remember that the interviewer is considering hiring you to oversee complex projects for their company. And, as they say, the proof is in the pudding. The interviewer wants assurance that you're capable of managing projects so give an example or two of past projects you've led that ended successfully. Clearly explain the details of the project, your role and actions, and the final result.