Years ago I was on a (dev) team and helping with interviews, and one question we'd ask sometimes was "What is your favorite piece of software, and why?" It was interesting because the reasons helped illustrate how applicants thought about software and what was important to them (UI, performance, community, features, etc etc etc)
I used to say Microsoft Flight Simulator, due to its enduring legacy and detailed simulation (and picking business software is yaaaaawn). But now I think Excel is my choice. What's yours?