With a question like this, the interviewer is gaining insight into how you approach and solve problems. It also provides an idea of the type of work you have already done. Be sure to explain the event, action, and result (EAR), avoid blaming others, and explain why this project was difficult:

“My most difficult project was on endangered animals. I had to predict how many animals would survive to 2020, 2050, and 2100. Before this, I’d dealt with data that was already there, with events that had already happened. So, I researched the various habitats, the animal’s predators and other factors, and did my predictions. I have high confidence in the results.”