I agree it's the best question because it's a good topic to begin a bigger conversation about your programming experience. It's hard to fake and you can take the discussion in many directions. Simply asking for their weirdest bug, getting an answer, and going "ok good, let's move on" is a bad interview question.

For example, you say the ratio of debugging:development changed dramatically over the course of your career. I would follow up and ask what key things you would attribute that to? Testing? Changing languages? Changing programming paradigms? Maybe it's simply that you have a wider knowledge of CS concepts and a stronger intuition for the correct way to model your logic. There's no right or wrong answers, just trying to see that you actually do have some opinions of your own.

I find the ratio doesn't change, but the trickiness of the bugs gets more complicated as you solve harder problems and avoid previously unknown problems.