Great question and good job being proactive with interview preparation. The goal of this question is really to give the interviewer an opportunity to learn more about you. You should feel free to talk any experience you're proud of (group project, internship, sports etc). There really is no wrong answer.
What's more important is how you frame your answer. After you've selected an experience, think about the qualities and traits you want to showcase.
Here some aspects you may want to highlight in your story:
- why is this achievement so special to you? is it something you are passionate and excited about?
Overcoming challenges and adversity
- Were there any challenges you faced? How did you overcome it?
- Did you ask for help?
- Were you resourceful, if so how?
- Did you come up with a creative solution? What was it and why was it unique?
- Did you research and find the right answer?
- Did you experiment and iteratively find an answer/solution?
Organization and Execution
- How did you organize yourself? Did you make a plan and execute against it?
- If it was a group effort, did you create deadline, milestones and a plan for success?
Communication and Collaboration
- Did you ask others for help?
- Did you work with others? How did you communicate and make sure everyone was on the same page.
Hopefully that's a useful staring point. There honestly is not right or typical answer. This an opportunity to showcase the best version of you. Be honest and provide details about why you are so proud of it.
Feel free to reply if you have a particular story in mind and would like to get additional feedback on how to best tell it. Good luck!