Generally it is the airplane you currently fly, or the most complex airplane you have flown. If you're a CFI and fly multiple airplanes, it will probably be the twin or most complex airplane.

However, if the interviewer has history with a certain airplane, that could come up too. I don't think it would be reasonable to ask systems Q's about an airplane from your past.

But if you're a CFI and go for a ride in a citation don't log SIC unless you're willing to break down that airplane at your next interview.