My answer: whichever one has the greatest financial impact. Interviewer answer: the one to please the highest ranking manager. Note that this was a panel interview, and the panel preferred answer. Note that this was 2006. The dropdown below only goes to 2010.
This was difficult since the job was not yet mine, and an idea is not copyrighted. If I shared too much and they did not hire me then they could still use it but if I did not share enough I risk losing the opportunity because I did not explain well enough.