This is kind of tricky question.Try to avoid saying anything negative in this question like always but if forced to answer what was the least rewarding, be very careful. You could name somethings that is part of your future new role. Try to glaze over this or mention something that was very specific to your last role and should not be part of your new position. You need to keep your answer positive. You could even frame it as something that was the least rewarding as compared to the most rewarding activities.