More important than the focal length is the field-of-view the lens gives with your camera. Short focal lengths give a wide field-of-view. They are often recommended for landscape photography because the field-of-view takes in a wide expanse of the landscape.
Long focal lengths give a narrow field-of-view, isolating just a small area of the scene. They are less suited to landscape work, but can be effective if you want to emphasise a particular aspect of the view, or eliminate less attractive areas. The interactive demo to show how focal length affects field-of-view and the resultant image.