What was your most challenging project and why?

I am asked this question now days by the interviewers. And it seems that they are not impressed by my answer. I am also not sure which project to tell them. Almost all of the projects were very interesting and challenging to me. I enjoyed working them and learned many things during work. Even when some task was boring like writing documentation, I made it challenging enough by automating it and didn't wrote the documentation myself but wrote the script that did it for me. What interviewers are looking for in this question? Can I tell more then one project? What was your most challenging project and why?