Seth Godin writes:

If someone asks you that, are you excited to tell them the answer?

in his post What are you working on? saying that you should be excited, even if it’s just your weekend project. I’m not that demanding, but I open all my interviews asking the candidate: “What’s the most exciting project you ever worked on?”. It can be years ago, I understand that sometimes we get stuck in shitty projects, but if you never managed to work in an exciting project I really worried.

How exciting? You should start talking, moving your hands around and forget you are in an interview. That exciting should be. Bonus points if I have to stop you because you just keep talking about the project and I have to do an interview.