If I were to contact your most recent supervisor, what area of your work would he/she tell me you could most improve upon?
When an interviewer asks this question, they aren't trying to make you feel uncomfortable, and they aren't looking to see how you assess your performance. Instead, they want to hear how you discuss your past supervisors and their demands. This allows them to get a better understanding of your outlook and attitude as an employee and your methods for discussing sensitive topics.
Choose an area that fits into the overall picture for the job. Focus on something that you recently learned, so you can show how you grow into tasks and what you do to improve as you acclimate. Be positive, but not overly confident.
"I think my boss would probably say I take on a bit more than I should, and I tend to get overwhelmed at times. We've discussed this, and I have developed a stronger sense of when to delegate, but I still do have ups and downs in my productivity because I occasionally over-schedule my time."
