Expressions are honestly my favorite part of drawing. It can be a lot of fun to make them super exaggerated, and play with body language as well. (Expressiveness also plays a sizeable role in deciding whether or not I like a comic.)
I think my favorites to draw are surprise and distress, like these:


//^This one is of course from a currently unfinished page
Also this vaugely creepy expression from an unfinished sketch: