Listening. I speak Spanish at about a B1/B2 level. When I travel in Latin America, I can read signs, menus, instructions, etc. pretty easily. I can make myself understood pretty easily, too. However, it is hard for me to understand spoken Spanish. When people speak to me, I look like a deer in the headlights. :)
It is more difficult in some countries than others. Of the 12 Spanish-speaking countries I have visited, Guatemalan and El Salvadoran Spanish was the easiest for me to understand, and Colombian the most difficult.