This question might seem like it is meant to trick you, but the interviewer is really looking for what types of things interest you the most as well as your willingness to be honest. Try to think about what it is that you have liked the most about your former jobs and decide what category they fall into. Topics for your favorite part of your job might be hands-on work, flexible schedule, working as part of a team, ability to affect change as far as policies and procedures, you felt your ideas were listened to, etc. As far as your least favorite part of your former job, try not to be too specific. Don't mention a specific supervisor or coworker by name. Topics for your least favorite part of your job might include strict overtime policy or unable to voice ideas and suggestions for updating policies and procedures.