This answer requires balance. A Senior Project Manager has to take care of, amongst many others, 5 key parameters: 1. People, 2. Deliverables, 3. Management, 4. Economics/Financial, 5. Technology. The best way to go about this answer is take up each of the above parameters one by one and describe the success you had in one or more of the above 5 parameters. You may also take a "star-project" of yours and describe it along the 5 parameters mentioned. Try to quantify wherever possible, explain the rationale why you (and possibly your peers and management) consider the described performance to be your achievement.