The most difficult questions are always the ones relating directly to yourself. At least, that's how I feel. When I was asked how others would explain my personality, I had to dig pretty deep. That doesn't necessarily mean that it was a genuinely difficult question. I just struggle with those types. Also, the common experience questions were asked, such as obstacles and challenges and how you deal with stress. The interview questions gave you a good idea of the requirements of the position, which made it easy to accept the job offer on the same day.