While some spots like MACBA or Fondo have a bunch of awesome places to skate in one spot others are more spread out like Forum. Montbau (named after the closest Metro station) is more like Forum. Here you will find several great spots all within a few blocks of each other. One of the more interesting ones is the Sculpture Park. Sculpture #1 provides two banks to skate with different steepness and you can even ride the sides which go into the sculpture. Great for pictures.
Sculpture #2 in right beside number one and is a little harder to skate but fun enough. Another block away and around the corner you will find a really weird looking brown sculpture. Not sure how good it is to skate but some people have definitely tried.
The next spot is the famous ledge that goes over a sand gap. While this ledge is fairly worn it provides another great photo op as well as some smooth pavement to cruise around on.