Hands and buildings! I can usually reference my own hand, or kidnap a friend or relative to act as hand model for me, but I've always struggled with the severe lines of buildings.
Vehicles: cars, carriages, trains etc. Also, guns.
Domestic cats. I don't know why, any other sort of cat is fine to draw, just something with domestic cats really screws me up. I also have difficulty drawing lips, but i'm currently working hard on that one.
ANIMALS. I can, but I need lots of references at all times.
Architecture, and Back Grounds that contain buildings, rooms, etc. hands down.
I would have to say Males. They don't have the curves like Females so it's quite hard to draw. I'm still practicing on drawing them. :)
Oh!! Animals are hard to draw too, especially horses. Still needs some practicing. :)
Something I've never drawn before! I tend to commit things to memory and be able to draw anything from that. I find different perspectives of the human head to be a challenge, as well as two people engaging in something such as kissing, and architecture. But I always work out the problem.
Trees. In full detail.
A look, hands and human anatomy in general. Because you always have to be very careful concerning proportions!
Hardest to draw is drawing without inspiration.
As long as I have a reference, nothing seems to difficult to draw. I find it easier to draw figures/objects than landscapes and atmospheres.
a city landscape
This is a question I often ask myself. While I have common subject matter in my commissioners, I try very hard not to "specialize" in any one thing. I feel comfortable drawing any subject matter primarily because of the way I taught myself. I would not simply get good at one thing and keep drawing it, if I didn't feel comfortable with something I would draw it until I was.