Question for leaders

As a potential leader, there are many questions that could be asked to determine your capability or capacity. The one I've never heard is this:

Aside from the people you've led and/or directed...of all the people you've mentored, who are you the most proud of?

It's my bet that many so-called leaders would be caught off-guard by this question because, in their quest to achieve personal goals and improve themselves, the notion of mentoring probably never caught their attention.