My most challenging thing was doing the last rewrite of the child care subsidy management system at our agency. It combined all the bits and pieces I had developed in the last decade, added several new layers of detail (was able to complete all forms and reports) and added depth (historical data tracking). Went from single user app (FoxBase - actually the old version used a distributed-data method so it was multi-user) to Web Based (LAMP) one. Well integrated...
There's just a lot to write about it, lots of different challenges and successes. and I could tell stories on those older systems too there are successes and challenges all the time.
I think a lot of this is demonstrating experience, interest and problem solving skills, might also touch on communication skills and able to relate technical aspects understandably.