It would be, like most people have said here, the hands and feet. I find drawing bare feet facing the front slightly difficult, though since most of the time I draw the characters with shoes on, or just chop off whats below the knee.

As for hands, since they're exposed most of the time, I find the positioning of the hands to be the difficult part since at times, I tend to draw the thumb facing the wrong direction, and it turns out very awkward.

The above two would be mainly for my anime drawings.

As for realism, I guess the most difficult one for me to do would be either the nose, lips of the clothes, since most of the time I do bust up pieces. The shading is really difficult at times, and I can't possibly seem to get the shading right for the lips, and sadly, it doesn't look correct at all. I have a difficult time shading clothes that have texture on them as well, cause I want to make them look as real as possible.

For the hands though, what would be difficult for me is doing the nails. @_@