People! At least, realistically. And anything that involves major perspective! Whenever we had to do those building/hallway perspective, I would audibly groan. I think it stems from my dislike of rulers.
For book illustration we had to pick a well known character and do a couple of sketch redesigns. And then we had to pick one of those designs and create a family for our character. I chose Rapunzel! Can you guys tell I draw people all of the time? Boys are hard to draw. Eaugh.